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'

递归法：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* swapPairs(ListNode* head) {
        if(!head||!head->next)
            return head;
        ListNode* p1=head,*p2=p1->next;
        ListNode* tmp1=p1->next->next;
        head=p2;
        p2->next=p1;
        p1->next=swapPairs(tmp1);
        return head;
    }
};