[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.

这道题有点意思。主要是记住交换后上一个的父节点还没改变呢，所以用递归来做是比较容易处理的。

class Solution {
public:
    ListNode *swapPairs(ListNode *head) {
        if(head==NULL || head->next==NULL)
            return head;
        ListNode *temp = head->next;
        ListNode *forward = temp->next;
        head->next->next = head;
        head->next=swapPairs(forward);
        return temp;
    }
};