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.

这题要求遍历链表，两两交换，也算是一道比较简单的题目，我们只需要拿到需要交换的前驱节点就可以了。直接上代码：

  1. class Solution {
  2. public:
  3.     ListNode *swapPairs(ListNode *head) {
  4.         if(!head || !head->next) {
  5.             return head;
  6.         }
  8.         ListNode dummy(0);
  9.         ListNode* p = &dummy;
  10.         dummy.next = head;
  12.         while(p && p->next && p->next->next) {
  13.             ListNode* n = p->next;
  14.             ListNode* nn = p->next->next;
  15.             p->next = nn;
  16.             n->next = nn->next;
  17.             nn->next = n;
  18.             p = p->next->next;
  19.         }
  21.         return dummy.next;
  22.     }
  23. };