【LeetCode】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.

题目要求不能只换val，需要将整个节点交换。用两个变量储存下就好了。

代码：

class Solution {
public:
	ListNode *swapPairs(ListNode *head) {
		ListNode *res = new ListNode(0);
		ListNode *node = res;
		while (head){
			ListNode *l1(NULL);
			ListNode *l2(NULL);
			if (head){
				l1 = head;
				head = head->next;
			}
			if (head){
				l2 = head;
				head = head->next;
			}
			if (l2){
				node->next = l2;
				node = node->next;
			}
			if (l1){
				node->next = l1;
				node = node->next;
			}
		}
		node->next=NULL;
		return res->next;
	}
};