24. 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​​.

not

加一个头结点

/**
 * 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 NULL;
    }
    ListNode dummy(0);
    dummy.next = head;
    ListNode* p = &dummy;
    while (p->next&&p->next->next){
      ListNode* next = p->next;
      ListNode* next2 = next->next;
      ListNode* next3 = next2->next;
      p->next = next2;
      next2->next = next;
      next->next = next3;
      p = next;
    }
    return dummy.next;
  }
};