24. Swap Nodes in Pairs.cpp

/*
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 1:
Input: head = [1,2,3,4]
Output: [2,1,4,3]

Example 2:
Input: head = []
Output: []

Example 3:
Input: head = [1]
Output: [1]

Constraints:
The number of nodes in the list is in the range [0, 100].
0 <= Node.val <= 100
*/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */

class Solution
{
public:
  ListNode *swapPairs(ListNode *head)
  {
    if (!head || !head->next)
      return head;

    auto next = swapPairs(head->next->next);
    auto curr = head->next;

    head->next = next;
    curr->next = head;
    head = curr;

    return head;
  }
};