Skip to main content

1539 - Kth Missing Positive Number (Easy)

https://leetcode.com/problems/kth-missing-positive-number/

Problem Statement​

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

Approach 1: Brute Force​

Iterate each number to find out the kk-th missing number.

Written by @wingkwong
class Solution {
public:
    int findKthPositive(vector<int>& arr, int k) {
        int target = 1;
        while (k > 0) {
            // target is missing. decrease k by 1
            if (find(arr.begin(), arr.end(), target) == arr.end()) k -= 1;
            // find the ans
            if (k == 0) break;
            // check next number
            target += 1;
        }
        return target;
    }
};