missingInteger.js

/*  Missing Integer - Codility
This is a demo task.

Write a function:

function solution(A);

that, given an array A of N integers, returns the smallest positive integer (greater than 0) that does not occur in A.

For example, given A = [1, 3, 6, 4, 1, 2], the function should return 5.

Given A = [1, 2, 3], the function should return 4.

Given A = [−1, −3], the function should return 1.

Assume that:

N is an integer within the range [1..100,000];
each element of array A is an integer within the range [−1,000,000..1,000,000].
Complexity:

expected worst-case time complexity is O(N);
expected worst-case space complexity is O(N) (not counting the storage required for input arguments).

*/

function solution(A) {
  A.sort(comparator);
  return A.reduce((prev, next) => {
    if (next > -1 && next === prev) {
      prev++;
    }
    return prev;
  }, 1);
}

// Sort the array explicit way
function comparator(a, b) {
  return a - b;
}

console.log(solution([1, 3, 6, 4, 1, 2])); // 5
console.log(solution([1, 2, 3])); // 4
console.log(solution([-1, -3])); // 1