Java [leetcode 24]Swap Nodes in Pairs

题目描述：

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.

解题思路：

我只想说递归大法好。

代码如下：

/**
 * 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)
			return null;
		if (head.next == null)
			return head;
		ListNode tmp = head.next;
		ListNode forward = head.next.next;
		tmp.next = head;
		head.next = swapPairs(forward);
		return tmp;
	}
}