[Algorithm] 21. Merge Two Sorted Lists

var mergeTwoLists = function(l1, l2) {
    if (!l1 && !l2) {
        return null;
    }
    
    if (!l1 && l2) {
        return l2;
    } 
    
    if (!l2 && l1) {
        return l1;
    }
    
    const smallerNode = l1.val <= l2.val ? l1 : l2;
    if (smallerNode.val === l1.val) {
        smallerNode.next = mergeTwoLists(l1.next, l2);
    } else {
        smallerNode.next = mergeTwoLists(l1, l2.next);
    }
    
    return smallerNode;
}

/**
 * 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) {

    let curr = temp = new ListNode(0)
    
    while (l1 && l2) {
        if (l1.val < l2.val) {
            curr.next = l1;
            l1 = l1.next;
        } else {
            curr.next = l2;
            l2 = l2.next;
        }
        curr = curr.next;
    }
    
    if (l1) {
        curr.next = l1;
    }
    
    if (l2) {
        curr.next = l2;
    }
    
    return temp.next;
};