06. Kth Missing Positive Number.cpp

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

Hint #1  
Keep track of how many positive numbers are missing as you scan the array.
*/

class Solution
{
public:
  int findKthPositive(vector<int> &arr, int k)
  {
    long long curr = 1, count = 0;
    int i = 0;

    while (count < k)
    {
      if (i < arr.size())
      {
        if (curr == arr[i])
        {
          i++;
        }
        else
          count++;
      }
      else
        count++;
      curr++;
    }

    return curr - 1;
  }
};