Merge In Between Linked Lists

/**
 * Definition for singly-linked list.
 * 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 last = list1;
        ListNode next = list1;
        
        
        while(last != null && a > 1){
            
            a--;
            last = last.next;
        }
        
        
        while(next != null && a <= b){
            
            a++;
            next = next.next;
        }
        
        last.next = list2;
        
        while(list2.next != null){
            list2 = list2.next;
        }
        
        list2.next = next.next;
        
        return list1;
    }
}