LeetCode 24. Swap Nodes in Pairs

24. Swap Nodes in Pairs

Difficulty:: Medium

Given a linked list, swap every two adjacent nodes and return its head.

You may not modify the values in the list’s nodes, only nodes itself may be changed.

Example:

1	Given 1->2->3->4, you should return the list as 2->1->4->3.

Solution

Language: Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *       int val;
 *       ListNode next;
 *       ListNode(int x) { val = x; }
 * }
 */
class Solution {
      public ListNode swapPairs(ListNode head) {
            if (head == null || head.next == null) {
                  return head;
           }
            ListNode dummy = new ListNode(0);
            dummy.next = head;
            head = dummy;
            
            ListNode prev = head;
            ListNode cur = head.next;
            while(cur != null && cur.next != null) {
                  ListNode next = cur.next;
                  ListNode tmp = cur.next.next;
                  cur.next.next = cur;
                  cur.next = tmp;
                  prev.next = next;
                  prev = cur;
                  cur = tmp;
           }
            return dummy.next;
     }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        if (head == null) {
            return null;
        }
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        head = dummy;
        while (head != null) {
            head = reverse(head);
        }
        return dummy.next;
    }
    
    private ListNode reverse(ListNode head) {
        if (head.next == null || head.next.next == null) {
            return null;
        }
        ListNode next = head.next;
        head.next = head.next.next;
        ListNode tmp = head.next.next;
        head.next.next = next;
        next.next = tmp;
        return next;
    }
}