This approach involves iterating through the array and maintaining a counter for missing positive integers. We start with the first positive number, which is 1, and determine if it is missing by comparing it to the current element in the array. If the number is missing, we decrement our k value. When k reaches zero, we have found our k-th missing number.
Time Complexity: O(n + k), where n is the length of the array. Space Complexity: O(1), as we are using constant extra space.
1public class Solution {
2 public int findKthPositive(int[] arr, int k) {
3 int missing_count = 0;
4 int current = 1;
5 int i = 0;
6 while (missing_count < k) {
7 if (i < arr.length && arr[i] == current) {
8 i++;
9 } else {
10 missing_count++;
11 if (missing_count == k) return current;
12 }
13 current++;
14 }
15 return -1; // this line is never reached
16 }
17 public static void main(String[] args) {
18 Solution solution = new Solution();
19 int[] arr = {2, 3, 4, 7, 11};
20 System.out.println(solution.findKthPositive(arr, 5));
21 }
22}This Java code follows the same logic as the C/C++ solutions. It maintains a missing count and iterates over the given array, checking missing numbers until the k-th missing number is found.
By using binary search, a more efficient solution can be developed that reduces unnecessary checks. The key observation is that the number of missing integers before arr[i] can be calculated as arr[i] - (i + 1), which helps in deducing the position of the k-th missing number without iterating through every integer.
Time Complexity: O(log n), Space Complexity: O(1).
1def findKthPositive(arr, k):
2 left, right = 0, len(arr) - 1
3 while left <= right:
4 mid = left + (right - left) // 2
5 if arr[mid] - (mid + 1) < k:
6 left = mid + 1
7 else:
8 right = mid - 1
9 return left + k
10
11arr = [2, 3, 4, 7, 11]
12k = 5
13print(findKthPositive(arr, k))The Python solution follows a similar binary search approach. It calculates the number of missing numbers up to each midpoint in the array and uses that to refine the search for the missing k-th integer.