1539. Kth Missing Positive Number-第 k 个缺失的正整数

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

Solution in python:

class Solution:
    def findKthPositive(self, arr: List[int], k: int) -> int:
        missing = []
        j = 0
        for i in range(1, arr[-1]):
            if i == arr[j]:
                j += 1
                continue
            else:
                missing.append(i)
        if k <= len(missing):
            return missing[k-1]
        else:
            return arr[-1]+k-len(missing)