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

void swap(ListNode* A, ListNode* B, ListNode* C, ListNode* last){
    A->next = C;
    B->next = A;
    last->next = B;
}

ListNode* solve(ListNode* input){
    ListNode *last = new ListNode(0);
    ListNode *ans = last;
    ListNode *A, *B, *C;
    last->next = input;
    while(last->next != NULL and last->next->next != NULL){
        A = last->next;
        B = A->next;
        C = B->next;
        swap(A, B, C, last);
        last = A;
    }
    return ans->next;
}
ListNode* Solution::swapPairs(ListNode* A) {
    if(A == NULL or A->next == NULL) return A;
    return solve(A);
}