leetcode 24. Swap Nodes in Pairs

题目内容

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.

题目分析

在链表中用到递归还是很方便的。本题可以通过递归来解决。

/**
* 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 second = head.next;
ListNode third = second.next;

second.next = head;
head.next = swapPairs(third);

return second;
}
}