LeetCode-24. Swap Nodes in Pairs

Given a linked list, swap every two adjacent nodes and return its head.

You may not modify the values in the list's nodes, only nodes itself may be changed.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

题解：

递归翻转

class Solution {
public:
  ListNode* swapPairs(ListNode* head) {
    if (head == NULL || head->next == NULL) {
      return head;
    }
    ListNode *p = head, *q = head->next;
    p->next = swapPairs(q->next);
    q->next = p;
    return q;
  }
};