題目：

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Note:

• Your algorithm should use only constant extra space.
• You may not modify the values in the list's nodes, only nodes itself may be changed.

程式碼：

#include<iostream>
#include<vector>

using namespace std;

struct ListNode {
	int val;
	ListNode *next;
	ListNode(int x) : val(x), next(NULL) {}
};

ListNode* swapPairs(ListNode* head) {

	if (!head) return NULL;
	ListNode* p = head;
	if (!p->next)return head;
	ListNode* q = p->next;
	ListNode* last = head;
	int i = 0;
	while (p&&q) {
		p->next = q->next;
		q->next = p;
		if (i == 0)head = q;
		else last->next = q;
		last = p;
		p = p->next;
		if (p) {
			q = p->next;
		}

		i++;
	}

	return head;
}

int main() {

	ListNode* l1 = new ListNode(1);
	ListNode* l2 = new ListNode(2);
	ListNode* l3 = new ListNode(3);
	ListNode* l4 = new ListNode(4);
	ListNode* l5 = new ListNode(5);
	ListNode* l6 = new ListNode(6);

	l1->next = l2;
	l2->next = l3;
	l3->next = l4;
	l4->next = l5;
	l5->next = l6;

	ListNode* l = swapPairs(l1);

	while (l) {
		cout << l->val << " ";
		l = l->next;
	}
	
	return 0;
}