开始刷leetcode day29：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.

Java:

/**
 * 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 newNode = head.next;
        head.next = head.next.next;
        newNode.next = head;
        head = head.next;
        ListNode newHead = newNode;
        newNode = newNode.next;
        
        while(head!= null && head.next != null)
        {
            newNode.next = head.next;
            head.next = head.next.next;
            newNode.next.next = head;
            newNode = head;
            head = head.next;
        }
        
        return newHead;
        
    }
}