1539. Kth Missing Positive Number

package LeetCode_1539

/**
 * 1539. Kth Missing Positive Number
 * https://leetcode.com/problems/kth-missing-positive-number/
 * Given an array arr of positive integers sorted in a strictly increasing order, and an integer k.
Find the kth 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. 1 <= arr.length <= 1000
2. 1 <= arr[i] <= 1000
3. 1 <= k <= 1000
4. arr[i] < arr[j] for 1 <= i < j <= arr.length
 * */
class Solution {
    /*
    * solution 1: Time:O(n), Space:O(1)
    * */
    fun findKthPositive(arr: IntArray, k: Int): Int {
        val n = arr.size
        //because i start from 1, so nums[i] = i + 1,
        for (i in arr.indices) {
            //when the gap >= k, that is the missing kth number
            if (arr[i]-(i+1)>=k){
                return k+i
            }
        }
        return n + k
    }
}