0024.cpp

/**
 * Definition for singly-linked list.
 * 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* dummy = new ListNode(NULL);
        dummy->next = head;

        int len = 0;
        for (auto curr = head; curr; curr = curr->next) len++;

        auto prev = dummy;
        auto curr = head;
        for (int i = 0; i < len / 2; i++) {
            auto next = curr->next;
            curr->next = next->next;
            next->next = prev->next;
            prev->next = next;
            prev = curr;
            curr = curr->next;
        }

        return dummy->next;
    }
};