Merge Two Lists LeetCode

Question

I've solved the problem on Repl.it website, but when I submitted the code on LeetCode it gave a typeError, that I'm going to paste it here:

Line 29 in solution.js
         throw new TypeError(__serialize__(ret) + " is not valid value for the expected return type 
ListNode");
         ^
TypeError: [] is not valid value for the expected return type ListNode
Line 29: Char 20 in solution.js (Object.<anonymous>)
Line 16: Char 8 in runner.js (Object.runner)
Line 13: Char 26 in solution.js (Object.<anonymous>)
Line 1200: Char 30 in loader.js (Module._compile)
Line 1220: Char 10 in loader.js (Object.Module._extensions..js)
Line 1049: Char 32 in loader.js (Module.load)
Line 937: Char 14 in loader.js (Function.Module._load)
at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
Line 17: Char 47 in run_main_module.js

And here's the code:

var mergeTwoLists = function(l1, l2) {
  let i = 0, j = 0;
  var out = [];
  while(i < l1.length || j < l2.length) {
    if(j == l2.length || i < l1.length && l1[i] < l2[j]) {
      out.push(l1[i++]);
    } else {
      out.push(l2[j++]);
    }
  }
  return out;
};

I really don't know where the problem is... if someone could help I would be grateful

Answer 1

This is a Linked List merge question, not a regular array merging. This'd pass through:

/**
 * 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) {
    var dummy = {
      val : -1,
      next : null
    };
    var curr = dummy;
    while (l1 && l2) {
        if (l1.val > l2.val) {
            curr.next = l2;
            l2 = l2.next;
        } else {
            curr.next = l1;
            l1 = l1.next;
        }
        curr = curr.next;
    }
    
    curr.next = l1 || l2;

    return dummy.next;
};

This is how your list would look like:

/**
 * 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}
 */

---

References

• For additional details, you can see the Discussion Board. There are plenty of accepted solutions with a variety of languages and explanations, efficient algorithms, as well as asymptotic time/space complexity analysis^{1, 2} in there.

If you are preparing for interviews:

• We'd write bug-free and clean codes based on standards and conventions (e.g., c^{1, 2}, c++^{1, 2}, java^{1, 2}, c#^{1, 2}, python¹, javascript¹, go¹, rust¹).