[Leetcode] Swap Nodes in Pairs

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.

链表神马的完全是我的克星啊。没有算法，好好画图，考虑p->next != NULL时一定要先考虑一下p!=NULL!!!!!!!!

/**
 * 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) return head;
        ListNode *h = new ListNode(-1), *pre = h, *cur = head;
        pre->next = head;
        ListNode *tmp;
        while (cur != NULL && cur->next != NULL) {
            tmp = cur->next;
            pre->next = tmp;
            cur->next = tmp->next;
            tmp->next = cur;
            pre = cur;
            cur = cur->next;
        }
        return h->next;
    }
};