24. Swap Nodes in Pairs

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

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

Your algorithm should use only constant space. You may not modify the values in the list, only nodes itself can be changed.

 

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
        if(head == null || head.next == null)
            return head;
        
        ListNode ret = head.next;
        
        ListNode first = head;
        ListNode second = head.next;
        
        while(second.next != null)
        {
            ListNode third = second.next;
            second.next = first;
            if(third.next == null)
            {
                first.next = third;
                return ret;
            }
            
            first.next = third.next;
            first = third;
            second = third.next;
        }
        
        if(second.next == null)
        {
            second.next = first;
            first.next = null;
        }
        
        return ret;
    }
}