merge-two-sorted-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} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function (l1, l2) {
    // Initialise a new LinkedList with a dummy ListNode
    let newList = new ListNode(0);

    // Maintain a reference to the head of the new LinkedList
    let headOfNewList = newList;

    // Whilst both of the passed in lists contain more elements
    while (l1 != null && l2 != null) {
        // If l1's value is smaller
        if (l1.val < l2.val) {
            // Add l1's value to the new list
            newList.next = l1;

            // Move l1 to its next element
            l1 = l1.next;
        } else {
            // Add l2's value to the new list
            newList.next = l2;

            // Move l2 to its next element
            l2 = l2.next;
        }

        // Move into the next level of the LinkedList for the next iteration
        newList = newList.next;
    }

    // If l1 has run out of elements
    if (l1 == null) {
        // Append l2 to the new list
        newList.next = l2;
    }
    // If l2 has run out of elements
    else {
        // Append l1 to the new list
        newList.next = l1;
    }

    return headOfNewList.next;
};