1539. Kth Missing Positive Number #

Problem #

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

Problem Statement #

Given a positive integer array arr sorted in strictly increasing order and an integer k. Please find the kth missing positive integer in this array.

Solution Approach #

• Easy problem. Use a variable starting from 1 and increment it, checking each time whether it exists in the array. If it does not exist, decrement k, until k becomes 0, at which point the current value is the value to output. Special case: all missing positive integers are outside the array, as in Example 2.

Code #

package leetcode

func findKthPositive(arr []int, k int) int {
	positive, index := 1, 0
	for index < len(arr) {
		if arr[index] != positive {
			k--
		} else {
			index++
		}
		if k == 0 {
			break
		}
		positive++
	}
	if k != 0 {
		positive += k - 1
	}
	return positive
}