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1539. Kth Missing Positive Number

Difficulty Topics

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

 

Follow up:

Could you solve this problem in less than O(n) complexity?

Solution

kth-missing-positive-number.py
class Solution:
    def findKthPositive(self, arr: List[int], k: int) -> int:
        start = 1

        s = set(arr)

        while k > 0 and start <= 1000:
            if start not in s: k -= 1

            start += 1

        if k > 0: start += k

        return start - 1