Swap Nodes in Pairs

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:

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

 

解法：先操作head，因为要返回的值。处理完之后就开始认真翻转。画画图就知道怎么转了，不难。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode swapPairs(ListNode head) {
        //先swap头两个
        if(head == null)return head;
        if(head.next == null)return head;
        if(head.next!=null){
            ListNode tmp = head; 
            ListNode tmpNext = head.next.next;
            head = head.next;
            head.next = tmp;
            head.next.next = tmpNext;
        }
        
        ListNode res = head.next;
        
        while(res !=null && res.next!=null && res.next.next!=null){ //这步判断是关键
            ListNode tmp = res.next;
            ListNode tmpNext = res.next.next.next;
            res.next = res.next.next;
            res.next.next = tmp;
            res.next.next.next = tmpNext;
            
            //res移位
            res = res.next.next;
        }
        return head;
        
    }
}