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Description:
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
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Solution:
class Solution {
public:
int findKthPositive(vector<int>& arr, int k) {
int l = 0;
int r = arr.size();
// find the first index l s.t. nMissing(l) = A[l] - l - 1 >= k
while (l < r) {
const int m = (l + r) / 2;
if (arr[m] - m - 1 >= k)
r = m;
else
l = m + 1;
}
// the k-th missing positive
// = A[l - 1] + k - nMissing(l - 1)
// = A[l - 1] + k - (A[l - 1] - (l - 1) - 1)
// = A[l - 1] + k - (A[l - 1] - l)
// = l + k
return l + k;
}
};
leetcode
challenge
Here is the link for the problem:
https://leetcode.com/problems/kth-missing-positive-number/
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