KthMissingPositiveNumber.java

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

class Solution {
    public int findKthPositive(int[] arr, int k) 
    {
        ArrayList<Integer> missing = new ArrayList<>();
        int number = 1;
        for(int i = 0; i < arr.length; i++)
        {
            while(number < arr[i])
                missing.add(number++);
            number++;
        }
        
        int answer = 0;
        while(missing.size() <= 1000)
            missing.add(number++);
        if(k<missing.size())
            answer = missing.get(k-1);
        return answer;
    }
}