LeetCode Hot 100 P24 Swap Nodes in Pairs

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Timeline

Timeline

2026-03-14

init

linked list

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Problem:

P24 Swap Nodes in Pairs

https://leetcode.cn/problems/swap-nodes-in-pairs/description/?envType=study-plan-v2&envId=top-100-liked

/**
 * 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) {}
 * };
 */

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 *virtual_head = new ListNode(0, head);
                ListNode *prev = virtual_head, *curr = prev->next, *next, *next_next;

                while (curr) {
                        next = curr->next;
                        if (next == nullptr) // Single node, no need to swap
                                break;
                        next_next = next->next;
                        prev->next = next;
                        next->next = curr;
                        curr->next = next_next;
                        prev = curr;
                        curr = next_next;
                }

                head = virtual_head->next;
                delete virtual_head;
                return head;
        }
};