https://practice.geeksforgeeks.org/problems/missing-number-in-array/0

missing-number-in-array.js

// https://practice.geeksforgeeks.org/problems/missing-number-in-array/0
function findMissingNum(arr) {
  const res = arr.sort((a,b) => a - b).filter((num, idx) => {
    return num !== idx + 1
  });
  return res.length ? res[0] - 1: -1;
}
const nums1 = [1,2,3,5];
const result1 = findMissingNum(nums1);
console.log(result1 === 4);
const nums2 = [1,2,3,4,5,6,7,8,10];
const result2 = findMissingNum(nums2);
console.log(result2 === 9);