LeetCode 1539. Kth Missing Positive Number

Description

https://leetcode.com/problems/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

Explanation

We can first find a list of missing positives from the list and then we have to handle different scenarios to find kth missing positive. If the list of missing positives has a length greater than k, returns the kth positive from the list. If the list of missing positives is empty, the k th missing positive would be the largest number in the list + k. If the missing positives’ length is shorter than k, the kth missing positive would be the largest number in the original list + k – len(missing_positives).

Python Solution

class Solution:
    def findKthPositive(self, arr: List[int], k: int) -> int:
        
        missing_positives = []
        
        
        n = max(arr)
        
        arr_set = set(arr)
        for i in range(1, n + 1):
            if i not in arr_set:
                missing_positives.append(i)
        
        
        if not missing_positives:
            return arr[len(arr) - 1] + k
        
        if k > len(missing_positives):
            return arr[len(arr) - 1] + (k - len(missing_positives))
        
        return missing_positives[k - 1]
        
  • Time Complexity: O(N).
  • Space Complexity: O(N).

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