1539. 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

Solutions

1. binary search

class Solution {
public:
    int findKthPositive(vector<int>& arr, int k) {
        if (arr[0] > k) return k;
        int lo = 0, hi = arr.size() - 1;
        while (lo < hi) {
            int mid = lo + ((hi - lo) >> 1);
            int numiss = arr[mid + 1] - (mid + 1 + 1);
            if (numiss < k)
                lo = mid + 1;
            else
                hi = mid;
        }
        return arr[lo] + (k - (arr[lo]  - (lo + 1)));
    }
};