1669-Merge-In-Between-Linked-Lists.js

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} list1
 * @param {number} a
 * @param {number} b
 * @param {ListNode} list2
 * @return {ListNode}
 */
var mergeInBetween = function(list1, a, b, list2) {
    let ptr = list1;
    for (let i = 0; i < a - 1; ++i)
        ptr = ptr.next;
    
    let qtr = ptr.next;
    for (let i = 0; i < b - a + 1; ++i)
        qtr = qtr.next;
    
    ptr.next = list2;
    while (list2.next)
        list2 = list2.next;
    list2.next = qtr;
    
    return list1;
};