Kth Missing Positive Number — LeetCode 1539 Python Solution

The 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

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.

Python solution

class Solution:
    def findKthPositive(self, arr: List[int], k: int) -> int:
        if arr[0] > k:
            return k
        left, right = 0, len(arr)
        while left < right:
            mid = (left + right) >> 1
            if arr[mid] - mid - 1 >= k:
                right = mid
            else:
                left = mid + 1
        return arr[left - 1] + k - (arr[left - 1] - (left - 1) - 1)

Complexity

Pattern: Monotonic Stack

Answer "what is the next greater element" for every position in one pass. LeetCode 1539. Kth Missing Positive Number is filed here because the reference solution below belongs to the algorithm family this hub collects, even though its LeetCode tags point elsewhere.

The monotonic stack guide has the Python template for the pattern and the 225 LeetCode problems that use it.

Related problems