Reverse Singly Linked List Java

Question

Can someone tell me why my code dosent work  I want to reverse a single linked list in java  This is the method  that doesnt work correctly   public void reverseList        Node before   null      Node tmp   head      Node next   tmp next      while tmp    null         if next    null           return        tmp next   before        before   tmp        tmp   next        next   next next            And this is the Node class   public class Node     public int data     public Node next     public Node int data  Node next         this data   data        this next   next           On input 4- 3- 2- 1 I got output 4  I debugged it and it sets pointers correctly but still I dont get why it outputs only 4

User · Answer

I don t get it    why not doing this     private LinkedList reverseLinkedList LinkedList originalList       LinkedList reversedList   new LinkedList lt  gt          for int i 0   i lt originalList size     i             reversedList add 0  originalList get i               return reversedList      I find this easier

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Using Recursion It s too easy    package com config   import java util Scanner   public class Help        public static void main String args              Scanner sc   new Scanner System in           Node head   null          Node temp   null          int choice   0          boolean flage   true          do              Node node   new Node                System out println  Enter Node                node data   sc nextInt                if flage                   head   node                  flage   false                            if temp  null                  temp next   node              temp   node               System out println  Enter 0 to exit                 choice   sc nextInt             while choice  0            Help getAll head            Node reverse   Help reverse head null             reverse   Help reverse head  null            Help getAll reverse               public static void getAll Node head           if head  null              return           System out println head data  Memory Add   head hashCode             getAll head next              public static Node reverse Node head Node tail           Node next   head next          head next   tail          return  next  null  reverse next head    head            class Node      int data   0      Node next   null

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If this isn t homework and you are doing this  manually  on purpose  then I would recommend using  Collections reverse list     Collections reverse   returns void  and your list is reversed after the call

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public class Linkedtest       public static void reverse List lt Object gt  list             int lenght   list size            for  int i   0  i  lt  lenght   2  i                  Object as   list get i               list set i  list get lenght - 1 - i                list set lenght - 1 - i  as                       public static void main String   args            LinkedList lt Object gt  st   new LinkedList lt Object gt             st add 1           st add 2           st add 3           st add 4           st add 5           Linkedtest reverse st           System out println  Reverse Value will be   st             This will be useful for any type of collection Object

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public void reverse         Node prev   null  Node current   head  Node next   current next      while current next    null            current next   prev          prev   current          current   next          next   current next            current next   prev      head   current

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You can also try this      LinkedListNode pointer   head      LinkedListNode prev   null  curr   null          Pointer variable loops through the LL        while pointer    null                   Proceed the pointer variable  Before that  store the current pointer             curr   pointer                       pointer   pointer next                       Reverse the link            curr next   prev              Current becomes previous for the next iteration            prev   curr                         System out println prev printForward

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I think your problem is that your initially last element next attribute isn t being changed becuase of your condition  if next    null       return    Is at the beginning of your loop   I would move it right after tmp next has been assigned   while tmp    null      tmp next   before    if next    null       return    before   tmp    tmp   next    next   next next

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public ListNode reverseList ListNode head        ListNode prev   null      ListNode curr   head      while  curr    null            ListNode nextTemp   curr next          curr next   prev          prev   curr          curr   nextTemp            return prev      check more details about complexity analysis http   javamicro com ref-card DS-Algo How-to-Reverse-Singly-Linked-List

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public class SinglyLinkedListImpl lt T gt         private Node lt T gt  head       public void add T element            Node lt T gt  item   new Node lt T gt  element           if  head    null                head   item            else               Node lt T gt  temp   head              while  temp next    null                    temp   temp next                            temp next   item                       private void reverse              Node lt T gt  temp   null          Node lt T gt  next   null          while  head    null                next   head next              head next   temp              temp   head              head   next                    head   temp             void printList Node lt T gt  node            while  node    null                System out print node data                     node   node next                    System out println               public static void main String a              SinglyLinkedListImpl lt Integer gt  sl   new SinglyLinkedListImpl lt Integer gt             sl add 1           sl add 2           sl add 3           sl add 4            sl printList sl head           sl reverse            sl printList sl head               static class Node lt T gt             private T data          private Node lt T gt  next           public Node T data                super                this data   data

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A more elegant solution would be to use recursion  void ReverseList ListNode current  ListNode previous                if current Next    null                                 ReverseList current Next  current                   ListNode temp   current Next                  temp Next   current                  current Next   previous

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I tried the below code and it works fine   Node head   firstNode  Node current   head  while current    null  amp  amp  current next    null       Node temp   current next      current next   temp next      temp next   head      head   temp      Basically one by one it sets the next pointer of one node to its next to next node  so from next onwards all nodes are attached at the back of the list

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public Node lt E gt  reverseList Node lt E gt  node        if  node    null    node next    null            return node            Node lt E gt  currentNode   node      Node lt E gt  previousNode   null      Node lt E gt  nextNode   null       while  currentNode    null            nextNode   currentNode next          currentNode next   previousNode          previousNode   currentNode          currentNode   nextNode            return previousNode

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I know the recursive solution is not the optimal one  but just wanted to add one here   public class LinkedListDemo        static class Node           int val          Node next           public Node int val  Node next                this val   val              this next   next                      Override         public String toString                 return      val                       public static void main String   args            Node n   new Node 1  new Node 2  new Node 3  new Node 20  null              display n           n   reverse n           display n              static Node reverse Node n            Node tail   n          while  tail next    null                tail   tail next                    reverseHelper n           return  tail              static Node reverseHelper Node n            if  n next    null                Node reverse   reverseHelper n next               reverse next   n              n next   null              return  n                     return  n              static void display Node n            for    n    null  n   n next                System out println n

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Reverse LinkedList     author asharda         class Node     int data    Node next    Node int data          this data data        public class ReverseLinkedList      static Node root    Node temp null    public void insert int data          if root  null              root new Node data              else             temp root        while temp next  null                  temp temp next                 Node newNode new Node data         temp next newNode             end of insert    public void display Node head          while head  null              System out println head data         head head next                public Node reverseLinkedList Node head          Node newNode      Node tempr null      while head  null              newNode new Node head data         newNode next tempr        tempr newNode        head head next            return tempr        public static void main String   args         ReverseLinkedList r new ReverseLinkedList        r insert 10       r insert 20       r insert 30       r display root       Node t r reverseLinkedList root       r display t

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To reverse a singly linked list you should have three nodes  top  beforeTop and AfterTop  Top is the header of singly linked list  hence beforeTop would be null and afterTop would be next element of top and with each iteration move forward  beforeTop is assigned top and top is assigned afterTop i e  top next    private static Node inverse Node top            Node beforeTop null  afterTop          while top  null               afterTop top next              top next beforeTop              beforeTop top              top afterTop                    return beforeTop

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Use this   if  current   null    current next  null  return current   Node nextItem   current next   current next   null   Node reverseRest   reverse nextItem    nextItem next   current   return reverseRest   or Java Program to reverse a Singly Linked List

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We can have three nodes previous current and next   public void reverseLinkedlist                   Have three nodes i e previousNode currentNode and nextNode  When currentNode is starting node  then previousNode will be null  Assign currentNode next to previousNode to reverse the link   In each iteration move currentNode and previousNode by  1 node               Node previousNode   null      Node currentNode   head      while  currentNode    null                 Node nextNode   currentNode next          currentNode next   previousNode          previousNode   currentNode          currentNode   nextNode            head   previousNode

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Java program for reversing the linked list     class LinkedList            static Node head           static class Node                int data              Node next               Node int d                    data   d                  next   null                                     Function to reverse the linked list          Node reverse Node node                Node prev   null              Node current   node              Node next   null              while  current    null                    next   current next                  current next   prev                  prev   current                  current   next                            node   prev              return node                           prints content of double linked list         void printList Node node                while  node    null                    System out print node data                         node   node next                                    public static void main String   args                LinkedList list   new LinkedList                list head   new Node 85               list head next   new Node 15               list head next next   new Node 4               list head next next next   new Node 20                System out println  Given Linked list                list printList head               head   list reverse head               System out println                  System out println  Reversed linked list                 list printList head                     OUTPUT  -  Given Linked list 85 15 4 20  Reversed linked list  20 4 15 85

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Node Reverse Node head            Node n rev          rev   new Node            rev data   head data          rev next   null            while head next    null               n   new Node                head   head next              n data   head data              n next   rev              rev   n              n null                  return rev      Use above function to reverse single linked list

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Node reverse rec Node start        if  start    null    start - gt  next    null           return start             Node new start   reverse start- gt next       start- gt next- gt next   start      start- gt next   null      return new start      Node reverse Node start        Node cur   start      Node bef   null       while  cur    null           Node nex   cur next         cur next   bef         bef   cur         cur   nex            return bef

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package com three   public class Link        int a      Link Next       public Link int i           a i            public class LinkList        Link First   null       public void insertFirst int a           Link objLink   new Link a            objLink Next First          First   objLink              public void displayLink             Link current   First          while current  null               System out println current a               current   current Next                        public void ReverseLink            Link current   First          Link Previous   null          Link temp   null           while current  null                if current  First                  temp   current Next              else                 temp current Next               if temp  null                   First   current                    return                            current Next Previous              Previous current                System out println Previous               current   temp                        public static void main String args              LinkList objLinkList   new LinkList            objLinkList insertFirst 1           objLinkList insertFirst 2           objLinkList insertFirst 3           objLinkList insertFirst 4           objLinkList insertFirst 5           objLinkList insertFirst 6           objLinkList insertFirst 7           objLinkList insertFirst 8           objLinkList displayLink            System out println  -----------------------------            objLinkList ReverseLink            objLinkList displayLink

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public static LinkedList reverseLinkedList LinkedList node        if  node    null    node getNext      null            return node             LinkedList remaining   reverseLinkedList node getNext         node getNext   setNext node       node setNext null       return remaining

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The method for reversing a linked list is as below   Reverse Method  public void reverseList         Node lt E gt  curr   head      Node lt E gt  pre   null      Node lt E gt  incoming   null       while curr    null            incoming   curr next       store incoming item          curr next   pre            swap nodes         pre   curr                 increment also pre          curr   incoming            increment current            head   pre     pre is the latest item where                    curr is null     Three references are needed to reverse a list  pre  curr  incoming           pre     curr    incoming     -- gt   n-1  -- gt   n  -- gt   n 1  -- gt        To reverse a node  you have to store previous element  so that you can use the simple stament   curr next   pre    To reverse the current element s direction  However  to iterate over the list  you have to store incoming element before the execution of the statement above because as reversing the current element s next reference  you don t know the incoming element anymore  that s why a third reference needed   The demo code is as below   LinkedList Sample Class  public class LinkedList lt E gt         protected Node lt E gt  head       public LinkedList             head   null             public LinkedList E   list            this            addAll list              public void addAll E   list            for int i   0  i  lt  list length  i                add list i               public void add E e            if head    null              head   new Node lt E gt  e           else               Node lt E gt  temp   head               while temp next    null                  temp   temp next               temp next   new Node lt E gt  e                        public void reverseList             Node lt E gt  curr   head          Node lt E gt  pre   null          Node lt E gt  incoming   null           while curr    null                incoming   curr next       store incoming item              curr next   pre            swap nodes             pre   curr                 increment also pre              curr   incoming            increment current                    head   pre     pre is the latest item where                        curr is null            public void printList             Node lt E gt  temp   head           System out print  List              while temp    null                System out print temp                     temp   temp next                     System out println               public static class Node lt E gt             protected E e          protected Node lt E gt  next           public Node E e                this e   e              this next   null                      Override         public String toString                 return e toString                          Test Code  public class ReverseLinkedList        public static void main String   args            Integer   list     4  3  2  1             LinkedList lt Integer gt  linkedList   new LinkedList lt Integer gt  list            linkedList printList            linkedList reverseList            linkedList printList               Output  List  4 3 2 1  List  1 2 3 4

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Node next   tmp next  while tmp    null     So what happens when tmp    null   You almost got it  though   Node before   null  Node tmp   head  while  tmp    null        Node next   tmp next      tmp next   before      before   tmp      tmp   next    head   before    Or in nicer     naming   Node reversedPart   null  Node current   head  while  current    null        Node next   current next      current next   reversedPart      reversedPart   current      current   next    head   reversedPart    ASCII art            lt    lt    lt         reversedPart      head                               head     current        gt    gt    gt

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public class ReverseLinkedList        public static void main String args             LinkedList lt String gt  linkedList   new LinkedList lt String gt             linkedList add  a            linkedList add  b            linkedList add  c            linkedList add  d            linkedList add  e            linkedList add  f            System out println  Original linkedList             for int i   0  i  lt  linkedList size  -1  i                  System out println   -    linkedList get i                       LinkedList lt String gt  reversedlinkedList   reverse linkedList           System out println  Reversed linkedList             for int i   0  i  lt  reversedlinkedList size  -1  i                 System out println   -    reversedlinkedList get i                          public static LinkedList lt String gt  reverse LinkedList lt String gt  linkedList            for int i   0  i  lt  linkedList size   2  i                 String temp   linkedList get i               linkedList set i  linkedList get linkedList size  -1-i                linkedList set  linkedList size  -1-i   temp                     return linkedList

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package LinkedList   import java util LinkedList   public class LinkedListNode        private int value      private LinkedListNode next   null       public LinkedListNode int i            this value   i             public LinkedListNode addNode int i            this next   new LinkedListNode i           return next             public LinkedListNode getNext             return next              Override     public String toString             String restElement   value  - gt            LinkedListNode newNext   getNext            while newNext    null               restElement   restElement   newNext value    - gt                newNext   newNext getNext             restElement   restElement  newNext          return restElement             public static void main String   args            LinkedListNode headnode   new LinkedListNode 1           headnode addNode 2  addNode 3  addNode 4  addNode 5  addNode 6            System out println headnode           headnode   reverse null headnode headnode getNext              System out println headnode              private static LinkedListNode reverse LinkedListNode prev  LinkedListNode current  LinkedListNode next            current setNext prev           if next    null              return current           return reverse current next next getNext                   private void setNext LinkedListNode prev            this next   prev

[java] Reverse Singly Linked List Java

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