Reversing an Array in Java

Question

If I have an array like this   1 4 9 16 9 7 4 9 11    What is the best way to reverse the array so that it looks like this   11 9 4 7 9 16 9 4 1    I have the code below  but I feel it is a little tedious   public int   reverse3 int   nums        return new int     nums 8   nums 7   nums 6   nums 5   num 4                          nums 3   nums 2   nums 1   nums 0         Is there a simpler way

User · Answer

If you don t want to use Collections then you can do this   for  i   0  i  lt  array length   2  i        int temp   array i     array i    array array length - 1 - i     array array length - 1 - i    temp

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In case you don t want to use a temporary variable  you can also do like this   final int len   arr length  for  int i 0  i  lt   len 2   i          arr i     arr len - 1 - i       a   a b     arr len - 1 - i    arr i  - arr len - 1 - i         b   a-b     arr i  -  arr len - 1 - i       a   a-b

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Or you could loop through it backeards  int   firstArray   new int   1 2 3 4   int   reversedArray   new int firstArray length   int j   0  for  int i   firstArray length -1  i  gt  0  i--       reversedArray j      firstArray i        note  I have not compiled this but hopefully it is correct

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you can send the original array to a method for example   after that you create a new array to hold the reversed elements  public static void reverse int   a    int   reversedArray   new int a length    for int i   0   i lt a length  i     reversedArray i    a a length -1 -i

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public void swap int   arr int a int b         int temp arr a       arr a  arr b       arr b  temp            public int   reverseArray int   arr       int size arr length-1       for int i 0 i lt size i              swap arr i size--                return arr

User · Answer

I like to keep the original array and return a copy  This is a generic version   public static  lt T gt  T   reverse T   array        T   copy   array clone        Collections reverse Arrays asList copy        return copy      without keeping the original array   public static  lt T gt  void reverse T   array        Collections reverse Arrays asList array

User · Answer

try this   public int   reverse3 int   nums        int   reversed   new int nums length       for  int i 0  i lt nums length  i              reversed i    nums nums length - 1 - i             return reversed      My input was   1  2  3  4  5  6  7  8  9  10  11  12  And the output I got   12  11  10  9  8  7  6  5  4  3  2  1

User · Answer

Collections reverse   can do that job for you if you put your numbers in a List of Integers   List lt Integer gt  list   Arrays asList 1  4  9  16  9  7  4  9  11   System out println list   Collections reverse list   System out println list     Output    1  4  9  16  9  7  4  9  11   11  9  4  7  9  16  9  4  1

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This code would help   int    a  1 2 3 4 5 6 7   for int i a length-1 i gt  0 i--    System out println a i

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In place reversal with minimum amount of swaps   for  int i   0  i  lt  a length   2  i          int tmp   a i       a i    a a length - 1 - i       a a length - 1 - i    tmp

User · Answer

you messed up  int   firstArray   new int   1 2 3 4   int   reversedArray   new int firstArray length   int j   0  for  int i   firstArray length -1  i  gt   0  i--       reversedArray j      firstArray i

User · Answer

The following will reverse in place the array between indexes i and j  to reverse the whole array call reverse a  0  a length - 1        public void reverse int   a  int i   int j            int ii    i          int jj   j           while  ii  lt  jj                swap ii  jj                 ii              --jj

User · Answer

You could use org apache commons lang ArrayUtils   ArrayUtils reverse array

User · Answer

If you want to reverse the array in-place   Collections reverse Arrays asList array      It works since Arrays asList returns a write-through proxy to the original array

User · Answer

I would do something like this   public int   reverse3 int   nums      int   numsReturn   new int nums length        int count   nums length  -1    for int num   nums        numsReturn count    num      count--        return numsReturn

[java] Reversing an Array in Java

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