24. Swap Nodes in Pairs

https://leetcode.com/problems/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

Solution

/**
 * 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 *current = head, *prev = nullptr;
        while (current != nullptr && current->next != nullptr) {
            ListNode *next = current->next;
            if (current == head) head = next;
            current->next = next->next;
            next->next = current;
            if (prev != nullptr) prev->next = next;
            prev = current;
            current = current->next;
        }
        return head;
    }
};

• #linkedlist