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 (57.98%)
 * Likes:    6347
 * Dislikes: 282
 * Total Accepted:    814K
 * Total Submissions: 1.4M
 * Testcase Example:  '[1,2,3,4]'
 *
 * Given a linked list, swap every two adjacent nodes and return its head. You
 * must solve the problem without modifying the values in the list's nodes
 * (i.e., only nodes themselves 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
 * 
 * 
 */

// @lc code=start
/**
 * 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) {
        ListNode *node = head;
        ListNode *previous = nullptr;
        while(node != nullptr)
        {
            if(node->next != nullptr)
            {
                ListNode *next = node->next;
                node->next = next->next;
                next->next = node;
                if(node == head)
                {
                    head = next;
                }
                if(previous != nullptr)
                {
                    previous->next = next;
                }
            }
            previous = node;
            node = node->next;
        }
        return head;
    }
};
// @lc code=end