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Problem

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

Return 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

Code

class Solution {
    // [2, 3, 4, 7, 11, 12] ->
    // [1, 1, 1, 3, 6, 6] 到每个位置时差了几个数字
    public int findKthPositive(int[] arr, int k) {
        int l = 0, r = arr.length;

        while (l < r) {
            int m = (l + r) / 2;
            if (arr[m] - m - 1 < k)
                l = m + 1;
            else
                r = m;
        }


        return l + k;
    }
}