24.swap-nodes-in-pairs.cpp

/*
 * @lc app=leetcode id=24 lang=cpp
 *
 * [24] Swap Nodes in Pairs
 *
 * https://leetcode.com/problems/swap-nodes-in-pairs/description/
 *
 * algorithms
 * Medium (43.24%)
 * Total Accepted:    283.5K
 * Total Submissions: 655.6K
 * Testcase Example:  '[1,2,3,4]'
 *
 * 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.
 *
 *
 */
#include <vector>
using namespace std;

struct ListNode
{
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};
class Solution
{
  public:
    ListNode *swapPairs(ListNode *head)
    {
        if (!head || !head->next)
            return head;
        ListNode *tmp = head->next;
        head->next = swapPairs(head->next->next);
        tmp->next = head;
        return tmp;
    }
};