Leetcode ---- Swap Nodes in Pairs

题目链接

题意：给出单链表头指针，要求交换链表中每一对相邻的结点．

注意：不可以改变链表中结点的值，只可以使用常量空间．

附上代码：

/**
 * 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 == NULL || head->next == NULL) {
            return head;
        }
        ListNode *p = head, *q = head->next, *pre = head;
        while (q != NULL) {
            p->next = q->next;
            q->next = p;
            if (pre == head) {
                head = q;
            } else {
                pre->next = q;
            }
            swap(q, p);
            pre = q;
            p = q->next;
            if (p == NULL) {
                break;
            } 
            q = q->next->next;
        }
        
        return head;
    }
};