LeetCode 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.

解法：可以使用递归，每次交换前两个，将剩下的链表递归，返回的是新的链表头。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode swapPairs(ListNode head) {
            if (head==null || head.next==null) {
                return head;
            }
        ListNode p = head;
        ListNode begin=null;
           ListNode temp = p;
           
           p=p.next;
           temp.next=p.next;
           p.next=temp;
           if (begin==null) {
            begin = p;
        }
            p=p.next;
            p.next=swapPairs(p.next);
            return begin;
    }
}