1539. Kth Missing Positive Number

Given an array arr of positive integers sorted in a strictly increasing order, and an integer k.

Find the k^th positive integer that is missing from this array.

Example 1:

Input: arr = [2,3,4,7,11], k = 5
Output: 9
Explanation: The missing positive integers are [1,5,6,8,9,10,12,13,...]. The 5th missing positive integer is 9.

Example 2:

Input: arr = [1,2,3,4], k = 2
Output: 6
Explanation: The missing positive integers are [5,6,7,...]. The 2nd missing positive integer is 6.

Constraints:

1 <= arr.length <= 1000
1 <= arr[i] <= 1000
1 <= k <= 1000
arr[i] < arr[j] for 1 <= i < j <= arr.length

/**
 * @param {number[]} arr
 * @param {number} k
 * @return {number}
 */
var findKthPositive = function(arr, k) {
  let i = 0,
    n = 1;
  while (k) {
    arr[i] != n ? k-- : i++;
    n++;
  }
  return n - 1;
};

Complexity Analysis

Time complexity: O(n)
Space complexity: O(1)