Merge Two Sorted List

mergeTwoSortedList.js

/*
Merge two sorted linked lists and return it as a sorted list. The list should be made by splicing together the nodes of the first two lists.
 
Example 1:
Input: l1 = [1,2,4], l2 = [1,3,4]
Output: [1,1,2,3,4,4]
Example 2:
Input: l1 = [], l2 = []
Output: []
Example 3:
Input: l1 = [], l2 = [0]
Output: [0]
 
Constraints:
The number of nodes in both lists is in the range [0, 50].
-100 <= Node.val <= 100
Both l1 and l2 are sorted in non-decreasing order.
*/

/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
 var mergeTwoLists = function(l1, l2) {
    let preHead = new ListNode(-1);
    
    let prev = preHead;
    
    while(l1 != null && l2 != null) {
        if (l1.val <= l2.val) {
            prev.next = l1;
            l1 = l1.next;
        } else {
            prev.next = l2;
            l2 = l2.next;
        }
        prev = prev.next;
    }
    
    // At least one of l1 and l2 can still have nodes at this point, so connect
    // the non-null list to the end of the merged list.
    prev.next = l1 === null ? l2 : l1;

    return preHead.next;
};