1539 - Kth Missing Positive Number

1539. Kth Missing Positive Number

Problem Statement

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?

Click to open Hints

• Keep track of how many positive numbers are missing as you scan the array.

Solutions

Rust

impl Solution {
    pub fn find_kth_positive(arr: Vec<i32>, k: i32) -> i32 {
        let (mut l, mut r) = (0, arr.len());

        while l < r {
            let mid = l + (r - l) / 2;

            if arr[mid] - mid as i32 - 1 >= k {
                r = mid;
            } else {
                l = mid + 1;
            }
        }

        l as i32 + k
    }
}

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