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The task is taken from LeetCode

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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

The following solutions are identical logically. They only differ in style:

Style 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) {
    if (!l1) return l2;
    if (!l2) return l1;
    if (l1.val < l2.val) {
        l1.next = mergeTwoLists(l1.next, l2);
        return l1;
    }
    l2.next = mergeTwoLists(l2.next, l1);
    return l2;
};

Style 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) {
    if (!l1 || !l2) return l1 || l2; 
    const linkThem = (smaller, greater) => {
        smaller.next = mergeTwoLists(smaller.next, greater);
        return smaller;
    };
    return l1.val < l2.val ? linkThem(l1,l2) : linkThem(l2,l1);
};
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