Merge Two Sorted Lists -- Leetcode

merge-two-sorted-lists.js

// https://leetcode.com/problems/merge-two-sorted-lists/submissions/
/**
 * 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 {ListNode} list2
 * @return {ListNode}
 */
var mergeTwoLists = function(list1, list2) {
    let n1 = list1;
    let n2 = list2;
    if (!n1 && !n2) {
        return null;
    } else if (!n1) {
        return n2;
    } else if (!n2) {
        return n1;
    }
    else if (n1.val >= n2.val) { // n2 (least) main
        while (n2 && n1) {
            while (n2.next && n1.val >= n2.next.val) {
                n2 = n2.next;
            }
            const tempn2next = n2.next;
            n2.next = n1;
            if (!tempn2next) {
                return list2;
            }
            while(n1.next && tempn2next && n1.next.val <= tempn2next.val) {
                n1 = n1.next;
            }
            const tempn1next = n1.next;
            n1.next = tempn2next; // join n1 to n2
            n1 = tempn1next;
            n2 = tempn2next;
        }
        return list2;
    } else { //n1 main
        while (n1 && n2) {
            while (n1.next && n1.next.val < n2.val) {
                n1 = n1.next;
            }
            const tempn1next = n1.next;
            n1.next = n2;
            if (!tempn1next) {
                return list1;
            }
            while(n2.next && tempn1next && n2.next.val <= tempn1next.val) {
                n2 = n2.next;
            }
            const tempn2next = n2.next;
            n2.next = tempn1next;
            n2 = tempn2next;
            n1 = n1.next;
        }
        return list1;
    }
};