0021-merge-two-sorted-lists.js

/**
 * https://leetcode.com/problems/merge-two-sorted-lists/
 * Time O(N + M) | Space O(N + M)
 * @param {ListNode} list1
 * @param {ListNode} list2
 * @return {ListNode}
 */
 var mergeTwoLists = function(list1, list2) {
    const isBaseCase1 = list1 === null;
    if (isBaseCase1) return list2;

    const isBaseCase2 = list2 === null;
    if (isBaseCase2) return list1;

    const isL2Greater = list1.val <= list2.val;
    if (isL2Greater) {
        list1.next = mergeTwoLists(list1.next, list2);/* Time O(N + M) | Space O(N + M) */

        return list1;
    }

    const isL2Less = list2.val <= list1.val;
    if (isL2Less) {
        list2.next = mergeTwoLists(list1, list2.next);/* Time O(N + M) | Space O(N + M) */

        return list2;
    }
}

/**
 * https://leetcode.com/problems/merge-two-sorted-lists/
 * Time O(N + M) | Space O(N + M)
 * @param {ListNode} list1
 * @param {ListNode} list2
 * @return {ListNode}
 */
var mergeTwoLists = function(list1, list2) {
    let sentinel = tail = new ListNode();

    while (list1 && list2) {/* Time O(N + M) */
        const isL2Greater = list1.val <= list2.val;
        const isL2Less = list2.val < list1.val;

        if (isL2Greater) {
            tail.next = list1;
            list1 = list1.next;
        }

        if (isL2Less) {
            tail.next = list2;
            list2 = list2.next;
        }

        tail = tail.next;
    }

    tail.next = list1 || list2;

    return sentinel.next;
};