How to write palindrome in JavaScript

Question

I wonder how to write palindrome in javascript  where I input different words and program shows if word is palindrome or not  For example word noon is palindrome  while bad is not   Thank you in advance

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This tests each end of the string going outside in  returning false as soon as a lack of symmetry is detected  For words with an odd number of letters  the center letter is not tested   function isPalindrome word       var head   0      var tail   word length - 1       while  head  lt  tail            if  word charAt head      word charAt tail                return false           else               head                 tail --                       return true

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Here is a 1 line program to check if your string is a palindrome  -     s    gt   Is   s  a palindrome    n Answer is  -   s split     reverse   join        s     bob     Just modify the last parameter of this IIFE to check different inputs  This is perhaps the best solution

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Faster Way   -Compute half the way in loop   -Store length of the word in a variable instead of calculating every time   EDIT  Store word length 2 in a temporary variable as not to calculate every time in the loop as pointed out by  mvw     function isPalindrome word      var i wLength   word length-1 wLengthToCompare   wLength 2      for  i   0  i  lt   wLengthToCompare   i           if  word charAt i     word charAt wLength-i             return false                 return true

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x000D   x000D  function palindrome str    x000D    var re      W   g     or var re      A-Za-z0-9  g  x000D    var lowRegStr   str toLowerCase   replace re       x000D    var reverseStr   lowRegStr split     reverse   join      x000D    return reverseStr     lowRegStr  x000D    x000D  console log palindrome  A nut for a jar of tuna     x000D  console log palindrome  Helo World     x000D   x000D   x000D

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How about this one    function pall  word         var lowerCWord   word toLowerCase        var rev   lowerCWord split     reverse   join           return rev startsWith lowerCWord          pall  Madam

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Here is an optimal and robust solution for checking string palindrome using ES6 features    x000D   x000D  const str  madam  x000D  var result     str  reduceRight  c v   gt   c v     str  Palindrome   Not Palindrome   x000D  console log result   x000D   x000D   x000D

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Here s a one-liner without using String reverse    const isPal   str   gt  Array    apply null  new Array strLen   str length      reduce  acc  s  i    gt  acc   str strLen -  i   1            str

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Best Way to check string is palindrome with more criteria like case and special characters     function checkPalindrom str        var str   str replace    a-zA-Z0-9   gi      toLowerCase        return str    str split     reverse   join          You can test it with following words and strings and gives you more specific result  1  bob 2  Doc  note  I dissent  A fast never prevents a fatness  I diet on cod  For strings it ignores special characters and convert string to lower case

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I think this implementation covers pretty much every case  I hope it s useful to someone   function isPalindrome word       if typeof word      undefined     word     null           console log  argument is undefined            return             if typeof word      string            word   word replace   s gi            if word length  lt  1           console log  wrong argument            return                 if word length     1           console log  It s palindrome             return                 for var i 0 i lt Math floor word length 2  i              if word i      word word length - 1 - i              console log  It s not palindrome              return                          console log  It s palindrome          else          console log  argument must be a string                And then you can try with these cases   isPalindrome  nurses run    isPalindrome  abcdcba    isPalindrome  hello world    isPalindrome        isPalindrome       isPalindrome      isPalindrome null   isPalindrome    isPalindrome 100   isPalindrome   name   John

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This function will remove all non-alphanumeric characters  punctuation  spaces  and symbols  and turn everything lower case in order to check for palindromes   function palindrome str        var re      A-Za-z0-9  g      str   str toLowerCase   replace re           return str    str split     reverse   join       true   false

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ES6 way of doing it  Notice that I take advantage of the array method reduceRight to reverse a string  you can use array methods on strings if you give the string as context  as low level - strings are arrays of chars   No it is not as performant as other solutions  but didn t see any answer that came it it using es6 or higher order functions so figured I d throw this one out there   const palindrome   str   gt      const middle   str length 2    const left   str slice 0  middle    const right   Array prototype reduceRight call str slice Math round middle     str  char    gt  str   char        return left     right

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Frist I valid this word with converting lowercase and removing whitespace and then compare with reverse word within parameter word   function isPalindrome input        const toValid   input trim     toLowerCase        const reverseWord   toValid split     reverse   join          return reverseWord    input toLowerCase   trim     true   false    isPalindrome      madam       true

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function palindrome str            var testString   str toLowerCase          var expression     a-z0-9  s  gi      if  testString match expression  reverse   join        testString match expression  join                return true        else           return false

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How can I verify if a number is palindrome    I have my function but is not right because it s for a string not a number   function palindrom str     for var i 0 i lt str length i         if str i    str str length-i-1          return false              return true     console log palindrom  121     console log palindrom  1234321     console log palindrom  3211432

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function isPalindrome str      var isPalindrome   true     if str length     2       return isPalindrome        for let i   0  d   str length - 1  i lt  d-i   i             if str i      str d - i           return  isPalindrome              return isPalindrome

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All these loops  How about some functional goodness    May run in to tail call issues on old current js engines  solved in ES6  function isPalendrome str       var valid   false      if str length  lt  2   return true      function even i ii           str i    str ii      i 1     ii    even i 1 ii-1    valid   true    null           function odd i  ii           str i    str ii      i     ii    odd i 1 ii-1    valid   true    null           if str length   2           return odd 0 str length-1  valid       else          return even 0 str length-1  valid            To test your call stack run this code  you will be able to parse strings double the call stack size  function checkStackSize        var runs   70000      var max process   1      var max   0      function recurse me             max process            if max process     runs  return          max   max process          try               recurse me             catch e                max    max process                      recurse me       console log max       Due to the symmetrical nature of the problem you could chunk the string from the outside in and process the chunks that are within call stack limits   by that I mean if the palindromes length is 1000  You could join 0-250 and 750-1000 and join 250-499 with 500-749  You can then pass each chunk in to the function  The advantage to this is you could run the process in parallel using web workers or threads for very large data sets

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function palindrome str        var lenMinusOne   str length - 1      var halfLen   Math floor str length   2        for  var i   0  i  lt  halfLen    i            if  str i     str lenMinusOne - i                 return false                      return true      Optimized for half string parsing and for constant value variables

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Taking a stab at this  Kind of hard to measure performance  though   function palin word        var i   0          len   word length - 1          max   word length   2   0       while  i  lt  max            if  word charCodeAt i      word charCodeAt len - i                 return false                    i    1            return true      My thinking is to use charCodeAt   instead charAt   with the hope that allocating a Number instead of a String will have better perf because Strings are variable length and might be more complex to allocate  Also  only iterating halfway through  as noted by sai  because that s all that s required  Also  if the length is odd  ex   aba    the middle character is always ok

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Here s another way of doing it   function isPalin str      str   str replace   W g     toLowerCase      return str  str split     reverse   join

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function palindrome str       for  var i   0  i  lt   str length  i              if   str i      str str length - 1 - i                 return  The string is not a palindrome                   return  The string IS a palindrome     palindrome  abcdcba       The string IS a palindrome  palindrome  abcdcb       The string is not a palindrome     If you console log this line   console log str i      and     str str length - 1 - i    before the if statement  you ll see what  str str length - 1 - i   is  I think this is the most confusing part but you ll get it easily when you check it out on your console

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str1  is the original string with deleted non-alphanumeric characters and spaces and str2 is the original string reversed   function palindrome str       var str1   str toLowerCase   replace   s g      replace         a-zA-Z 0-9  gi          var str2   str toLowerCase   replace   s g      replace         a-zA-Z 0-9  gi      split     reverse   join          if  str1     str2        return true        return false     palindrome  almostomla

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Below code tells how to get a string from textBox and tell you whether it is a palindrome are not  amp  displays your answer in another textbox       lt html gt   lt head gt   lt meta charset  UTF-8   gt   lt link rel  stylesheet  href     gt    lt  head gt     lt body gt      lt h1 gt 1234 lt  h1 gt   lt div id  demo  gt Example lt  div gt   lt a  accessKey  x  href  http   www google com  id  com   gt GooGle lt  a gt   lt h1 id  tar  gt  This is a Example Text     lt  h1 gt  Number1    lt input type  text  name  txtname  id  numb   gt  Number2    lt input type  text  name  txtname2  id  numb2   gt  Number2    lt input type  text  name  txtname3  id  numb3    gt   lt button type  submit   id  sum  onclick  myfun     gt count lt  button gt   lt button type  button   id  so2  onclick  div     gt counnt lt  button gt  lt br  gt  lt br  gt   lt ol gt   lt li gt water lt  li gt   lt li gt Mazaa lt  li gt   lt  ol gt  lt br  gt  lt br  gt   lt button onclick  myfun    gt TryMe lt  button gt       lt script gt      function myfun        var pass   document getElementById  numb   value      var rev   pass split     reverse   join          var text   document getElementById  numb3        text value   rev      if pass     rev       alert pass     is a Palindrome         else      alert pass     is Not a Palindrome                    lt  script gt   lt  body gt     lt  html gt

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Following function does the below mentioned task    1  Fetch all the substrings of a given string    2  Find which of the substrings are palindromes   3  Find the longest palindrome   4  Find if the longest palindrome is a lucky palindrome         function SubstringMnipulations S        let Str   S toString         let AllSubStrs   splitSubstrings Str       let Pelindromes   arePelindrome AllSubStrs       let LongestPelindrome   getLongestString Pelindromes       let isPrimeVar   isPrime LongestPelindrome       return          possibleSubstrings   AllSubStrs         pelindromes   Pelindromes         longestPelindrome   LongestPelindrome         isLuckyPelindrome   isPrimeVar             function splitSubstrings Str      let StrLength   Str length    let maxIndex   StrLength    let AllSubStrs         for var index   0  index  lt  maxIndex  index           for var innerIndex   1  innerIndex  lt   maxIndex-index  innerIndex            AllSubStrs push Str substring index   index innerIndex                 return AllSubStrs     function arePelindrome StrArr      let pelindrome         for i 0  i lt StrArr length i          if isPelindrome StrArr i            if StrArr i  length gt 1            pelindrome push StrArr i                         return pelindrome     function isPelindrome Str      let strLen   Str length    let firstHalfIndex   0    if strLen  1        return true         let firstHalf         let secondHalf          if strLen gt 1  amp  amp  strLen 2  0        firstHalfIndex   strLen 2      firstHalf   Str substring 0  firstHalfIndex       secondHalf   Str substring  firstHalfIndex   strLen       else       firstHalfIndex    strLen-1  2      firstHalf   Str substring 0  firstHalfIndex       secondHalf   Str substring  1 firstHalfIndex   strLen          secondHalf   reverseString secondHalf     return firstHalf     secondHalf     function reverseString Str      return Str split     reverse   join         function getLongestString Str      let lengthCount         for i 0  i lt Str length i          lengthCount push Str i  length          return Str lengthCount indexOf Math max    lengthCount        function isPrime Str      let input   Str length    let prime   true      for  let i   2  i  lt   Math sqrt input   i              if  input   i    0                prime   false              break                      return prime  amp  amp   input  gt  1

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A simple one line code to check whether the string is palindrome or not    x000D   x000D  function palindrome  str    x000D   x000D    return str     str split     reverse   join      x000D   x000D    x000D   lt  -- Change the argument to check for other strings -- gt  x000D   lt button type  button  onclick  alert palindrome  naman      gt Click Me lt utton gt  x000D   x000D   x000D    The above function will return true if the string is palindrome  Else  it will return false

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Use something like this  function isPalindrome s        return s    s split     reverse   join       true   false     alert isPalindrome  noon       alternatively the above code can be optimized as  updated after rightfold s comment   function isPalindrome s        return s    s split     reverse   join         alert isPalindrome  malayalam      alert isPalindrome  english

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The code is concise quick fast and understandable    TL DR  Explanation     Here isPalindrome function accepts a str parameter which is typeof string     If the length of the str param is less than or equal to one it simply returns  false    If the above case is false then it moves on to the second if statement and checks that if the character at 0 position of the string is same as character at the last place  It does an inequality test between the both    str charAt 0      gives us the value of character in string at position 0 str slice -1      gives us the value of last character in the string     If the inequality result is true then it goes ahead and returns false     If result from the previous statement is false then it recursively calls the isPalindrome str  function over and over again until the final result      x000D   x000D   function isPalindrome str   x000D    x000D   if  str length  lt   1  return true  x000D   if  str charAt 0     str slice -1   return false  x000D   return isPalindrome str substring 1 str length-1    x000D      x000D   x000D   x000D  document getElementById  submit   addEventListener  click  function    x000D   var str   prompt  whats the string     x000D   alert isPalindrome str   x000D      x000D   x000D  document getElementById  ispdrm   onsubmit   function   alert isPalindrome document getElementById  inputTxt   value    x000D    x000D   lt  DOCTYPE html gt  x000D   lt html gt  x000D   lt body gt  x000D   lt form id  ispdrm  gt  lt input type  text  id  inputTxt  gt  lt  form gt  x000D   x000D   lt button id  submit  gt Click me lt  button gt  x000D   lt  body gt  x000D   lt  html gt  x000D   x000D   x000D

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function isPalindrome s        return s    reverseString s      console log  isPalindrome  abcba       function reverseString str       let finalStr        for let i str length-1 i gt  0 i--           finalStr    str i            return finalStr

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Let us start from the recursive definition of a palindrome    The empty string    is a palindrome The string consisting of the character c  thus  c   is a palindrome If the string s is a palindrome  then the string  c    s    c  for some character c is a palindrome    This definition can be coded straight into JavaScript   function isPalindrome s      var len   s length       definition clauses 1  and 2    if  len  lt  2        return true           note  len  gt   2      definition clause 3    if  s 0     s len - 1         return false           note  string is of form s    a    t    a       note  s length  gt   2 implies t length  gt   0   var t   s substr 1  len - 2     return isPalindrome t       Here is some additional test code for MongoDB s mongo JavaScript shell  in a web browser with debugger replace print   with console log    function test s      print  isPalindrome     s           isPalindrome s       test      test  a    test  ab    test  aa    test  aab    test  aba    test  aaa    test  abaa    test  neilarmstronggnortsmralien    test  neilarmstrongxgnortsmralien    test  neilarmstrongxsortsmralien      I got this output     mongo palindrome js MongoDB shell version  2 4 8 connecting to  test isPalindrome    true isPalindrome a   true isPalindrome ab   false isPalindrome aa   true isPalindrome aab   false isPalindrome aba   true isPalindrome aaa   true isPalindrome abaa   false isPalindrome neilarmstronggnortsmralien   true isPalindrome neilarmstrongxgnortsmralien   true isPalindrome neilarmstrongxsortsmralien   false   An iterative solution is   function isPalindrome s      var len   s length    if  len  lt  2        return true        var i   0    var j   len - 1    while  i  lt  j        if  s i     s j           return false            i    1      j -  1        return true

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25x faster   recursive   non-branching   terse  function isPalindrome s i     return  i i  0  lt 0  i gt  s length gt  gt 1  s i   s s length-1-i  amp  amp isPalindrome s   i       See my complete explanation here

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Note  This is case sensitive  function palindrome word        for var i 0 i lt word length 2 i            if word charAt i   word charAt word length- i 1                return word   is Not a Palindrome       return word   is Palindrome       Here is the fiddle  http   jsfiddle net eJx4v

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I think following function with time complexity of o log n  will be better       function palindrom s       s   s toString        var f   true  l   s length 2  len   s length -1      for var i 0  i  lt  l  i                 if s i     s len - i                        f   false                       break                          return f           console log palindrom 12321

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To avoid errors with special characters use this function below  function palindrome str     var removeChar   str replace    A-Z0-9  ig      toLowerCase      var checkPalindrome   removeChar split     reverse   join        if removeChar     checkPalindrome        return true     else      return false

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My short solution  with the length of 66 characters  The result  true or false    x000D   x000D  function isPalindrome s    x000D    s s toLowerCase   match   w g   x000D    return s join    s reverse   join    x000D    x000D   x000D   x000D

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function isPalindrome word        let strLen   word length      word   word toLocaleLowerCase         if  strLen     0    strLen     1            return true             if  word 0      word strLen - 1              return isPalindrome word slice 1  strLen - 1                  return false

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What i use -  function isPalindrome arg       for let i 0 i lt arg length i             if arg i    arg  arg length-1 -i              return false      return true

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Try this   var isPalindrome   function  string        if  string    string split     reverse   join                alert string     is palindrome               else           alert string     is not palindrome              document getElementById  form id   onsubmit   function        isPalindrome document getElementById  your input   value       So this script alerts the result  is it palindrome or not  You need to change the your id with your input id and form id with your form id to get this work   Demo

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function Palindrome str      let forwardStr   str toLowerCase   replace    W   g         let reversedStr   forwardStr split     reverse   join       return forwardStr     reversedStr    console log Palindrome  madam

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function palindrome s      var re      W   g    var lowRegStr   s toLowerCase   replace re         var reverseStr   lowRegStr split     reverse   join         return reverseStr     lowRegStr

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Or you could do it like this   var palindrome   word   gt  word    word split     reverse   join

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You can try the following code with n 2 complexity    palindrom word               let len   word length          let limit   len   2    len - 1  2    len 2           for let i 0  i  lt  limit  i                          if word i     word len-i-1                                 alert  non a palindrom                   return                                  alert  palindrom           return        calling the function palindrom  abba

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Look at this   function isPalindrome word       if word  null    word length  0              up to you if you want true or false here  don t comment saying you             would put true  I put this check here because of             the following i  lt  Math ceil word length 2   amp  amp  i lt  word length         return false            var lastIndex Math ceil word length 2       for  var i   0  i  lt  lastIndex   amp  amp  i lt  word length  i              if  word i     word word length-1-i                 return false                        return true       Edit  now half operation of comparison are performed since I iterate only up to half word to compare it with the last part of the word  Faster for large data     Since the string is an array of char no need to use charAt functions     Reference  http   wiki answers com Q Javascript code for palindrome

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function palindrome str            var re      A-Za-z0-9  g          str   str toLowerCase   replace re               var len   str length          for  var i   0  i  lt  len 2  i                  if  str i      str len - 1 - i                     return false                                  return true

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For better performance you can also use this one  function palindrome str     str   str split       var i   str length   var check    Yes   if  i  gt  1        for  var j   0  j  lt  i   2  j              if  str j     str i - 1 - j                 check    NO               break                                 console log check         else       console log  YES

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Alternative solution for using Array prototype every    function palindrome str      return str split     every  char  index    gt         return char     str str length - index - 1

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function palindrome str         var len   str length      var mid   Math floor len 2        for   var i   0  i  lt  mid  i               if  str i      str len - 1 - i                 return false                       return true     palindrome will return if specified word is palindrome  based on boolean value  true false  UPDATE  I opened bounty on this question due to performance and I ve done research and here are the results  If we are dealing with very large amount of data like var abc    quot 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quot    for   var i   0  i  lt  10  i           abc    abc      making string even more larger    function reverse s       using this method for second half of string to be embedded     return s split  quot  quot   reverse   join  quot  quot       abc    reverse abc      adding second half string to make string true palindrome  In this example palindrome is True  just to note Posted palindrome function gives us time from 180 to 210 Milliseconds  in current example   and the function posted below with string    string split     reverse   join     method gives us 980 to 1010 Milliseconds   Machine Details  System       Ubuntu 13 10 OS Type      32 Bit RAM          2 Gb CPU          3 4 Ghz 2 Browser      Firefox 27 0 1

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x000D   x000D      function reversName name   x000D              return name x000D                     split     x000D                     reverse   x000D                     join         name   x000D         x000D      console log reversName  suus     x000D      console log reversName  suusss     x000D   x000D   x000D

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Try this   isPalindrome    string    gt        if  string     string split     reverse   join                console log  is palindrome              else           console log  is not palindrome             isPalindrome string

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You could also do something like this     function isPalindrome str    var newStr        for var i   str length - 1  i  gt  0  i--        newStr    str i      if newStr    str        return true      return newStr    else       return false      return newStr

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I am not sure how this JSPerf check the code performance  I just tried to reverse the string  amp  check the values  Please comment about the Pros  amp  Cons of this method   function palindrome str        var re   str split              reArr   re slice 0  reverse         for  a   0  a  lt  re length  a              if  re a     reArr a                 return false            else               return true                      JS Perf test

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The most important thing to do when solving a Technical Test is Don t use shortcut methods -- they want to see how you think algorithmically  Not your use of methods   Here is one that I came up with  45 minutes after I blew the test   There are a couple optimizations to make though  When writing any algorithm  its best to assume false and alter the logic if its looking to be true   isPalindrome     Basically  to make this run in O N   linear  complexity you want to have 2 iterators whose vectors point towards each other  Meaning  one iterator that starts at the beginning and one that starts at the end  each traveling inward  You could have the iterators traverse the whole array and use a condition to break return once they meet in the middle  but it may save some work to only give each iterator a half-length by default   for loops seem to force the use of more checks  so I used while loops - which I m less comfortable with   Here s the code          TODO  If func counts out  let it return 0       Assume  isPalindrome  invert logic      function isPalindrome S       var s   S         len   s length         mid   len 2          i   0  j   len-1       while i lt mid           var l   s charAt i           while j gt  mid               var r   s charAt j               if l     r                   console log   while     i  l         j  r                   --j                  break                            console log   while     i  l         j  r               return 0                      i            return 1     var nooe   solution  neveroddoreven        even char length var kayak   solution  kayak        odd char length var kayaks   solution  kayaks     console log   isPalindrome   nooe  kayak  kayaks     Notice that if the loops count out  it returns true  All the logic should be inverted so that it by default returns false  I also used one short cut method String prototype charAt n   but I felt OK with this as every language natively supports this method

[javascript] How to write palindrome in JavaScript

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