/**
 * 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* mergeInBetween(ListNode* list1, int a, int b, ListNode* list2) {
        ListNode *start, *end;
        start = list1; end = list1;
        a--;
        while(a || b){
            if(a) start = start->next, a--;
            if(b) end = end->next, b--;
        }
        start->next = list2;
        while(list2->next != nullptr){
            list2 = list2->next;
        }
        list2->next = end->next;
        return list1;
    }
};