21. Merge Two Sorted Lists

Description

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Solution

Solution 1

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function(l1, l2) {
    "use strict";
    let head = new ListNode(0);
    let ptr = head;
    while (l1 && l2) {
        if (l1.val < l2.val) {
            ptr.next = l1;
            l1 = l1.next;
        } else {
            ptr.next = l2;
            l2 = l2.next;
        }
        ptr = ptr.next;
    }
    ptr.next = l1 ? l1 : l2;
    return head.next;
};

Solution 2

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function(l1, l2) {
    "use strict";
    if (l1 && l2) {
        if (l1.val < l2.val) {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    } else if (l1) {
        return l1;
    } else {
        return l2;
    }
};