1669. Merge in between linked lists

//1669. Merge in between linked lists

 public class ListNode {
     int val;
     ListNode next;
     ListNode() {}
     ListNode(int val) { this.val = val; }
     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}

class Solution {
    public ListNode mergeInBetween(ListNode list1, int a, int b, ListNode list2) {
        ListNode end = list1;
        ListNode start = null;
        int i;

        for(i = 0; i < b; i++) {
            if(i == (a - 1)) {
                start = end;
            }

            end = end.next;
        }

        start.next = list2;

        while(list2.next != null) {
            list2 = list2.next;
        }

        list2.next = end.next;
        end.next = null;

        return list1;
    }
}