Task 76. UniLecs. Check minimal missing value of the permutation. Only natural numbers are considered. The size of the input array is not more than 10000.

permutation.js

const permutation = (arr) => {
  let max
  const set = []

  // set maximum and set
  for (let i = 0; i < arr.length; i++) {
    const element = arr[i]
    // update maximum of elements
    if (!max || max < element) {
      max = element
    }
    // set the element is present
    if (!set[element]) {
      set[element] = true
    }
  }

  // find missing element
  for (let i = 1; i < max; i++) {
    if (!set[i]) {
      return i
    }
  }
  // if not found, return 0
  return 0
}

console.log(permutation([ 1, 4, 2, 5, 6 ])) // should be 3 
console.log(permutation([ 4, 2, 5, 6 ])) // should be 1, not 3 (1 < 3) 
console.log(permutation([ 2, 5, 6 ])) // should be also 1, not 3 or 4 (1 < 3, 4) 
console.log(permutation([ 1, 4, 3, 2, 6, 5 ])) // should be 0
console.log(permutation([ 1, 8 ])) // should be 2 (2 < 3, 4, 5, 6, 7)