【LeetCode OJ】Swap Nodes in Pairs

题目：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.

代码：

struct ListNode
{
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
    
};
ListNode *swapPairs(ListNode *head)
{
    if (head == NULL || head->next == NULL)
        return head;
    ListNode *p = head;
    ListNode *q = p->next;
    while (q)
    {
        int temp;
        temp = p->val;
        p->val = q->val;
        q->val = temp;
        if (q->next != NULL)
        {
            p = q->next;
            if (p->next != NULL)
                q = p->next;
            else
                break;
            
        }
            else
                break;
        }
        return head;
    }