Missing Integer – Codility 100% Correct Javascript Solution

The Missing Integer problem in the 4th Codility lesson is marked as medium but I think the solution is pretty straight forward.

The complexity of my solution is O(N) or O(N * log(N)).

First, we check which is the max value of the array. If it is below 0 we immediately return 1.

We then fill an array of length max with the number of occurrences of each positive element from the A array.

Lastly, we check for the first appearance of 0 in the array of occurrence and return the value we were looking for.

.function solution(A) {
  let max = Math.max.apply(null,A);
  if (max < 0) return 1;
  let B = new Array(max).fill(0);
  for (let i = 0; i < A.length; i++) {
    if (A[i] > 0) {
      B[A[i] - 1]++;
  let index = B.indexOf(0);
  if ((index === -1)) {
    return max + 1;
  } else {
    return index + 1;

Click here to see the report of this algo.

Leave a Reply