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.

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

leetcode - Swap Nodes in Pairs