freecodecamp--freecodecamp/curriculum/challenges/english/blocks/daily-coding-challenges-javascript/68af0687ef34c76c28ffa54f.md

id, title, challengeType, dashedName

id	title	challengeType	dashedName
68af0687ef34c76c28ffa54f	Challenge 34: Missing Numbers	28	challenge-34

--description--

Given an array of integers from 1 to n, inclusive, return an array of all the missing integers between 1 and n (where n is the largest number in the given array).

• The given array may be unsorted and may contain duplicates.
• The returned array should be in ascending order.
• If no integers are missing, return an empty array.

--hints--

findMissingNumbers([1, 3, 5]) should return [2, 4].

assert.deepEqual(findMissingNumbers([1, 3, 5]), [2, 4]);

findMissingNumbers([1, 2, 3, 4, 5]) should return [].

assert.deepEqual(findMissingNumbers([1, 2, 3, 4, 5]), []);

findMissingNumbers([1, 10]) should return [2, 3, 4, 5, 6, 7, 8, 9].

assert.deepEqual(findMissingNumbers([1, 10]), [2, 3, 4, 5, 6, 7, 8, 9]);

findMissingNumbers([10, 1, 10, 1, 10, 1]) should return [2, 3, 4, 5, 6, 7, 8, 9].

assert.deepEqual(findMissingNumbers([10, 1, 10, 1, 10, 1]), [2, 3, 4, 5, 6, 7, 8, 9]);

findMissingNumbers([3, 1, 4, 1, 5, 9]) should return [2, 6, 7, 8].

assert.deepEqual(findMissingNumbers([1, 2, 3, 4, 5]), []);

findMissingNumbers([1, 2, 3, 4, 5, 7, 8, 9, 10, 12, 6, 8, 9, 3, 2, 10, 7, 4]) should return [11].

assert.deepEqual(findMissingNumbers([1, 2, 3, 4, 5, 7, 8, 9, 10, 12, 6, 8, 9, 3, 2, 10, 7, 4]), [11]);

--seed--

--seed-contents--

function findMissingNumbers(arr) {

  return arr;
}

--solutions--

function findMissingNumbers(arr) {
  if (arr.length === 0) return [];
  
  const n = Math.max(...arr);
  const set = new Set(arr);
  const result = [];

  for (let i = 1; i <= n; i++) {
    if (!set.has(i)) {
      result.push(i);
    }
  }
  return result;
}