K-th smallest element after removing some integers from natural numbers in C++

In this tutorial, we are going to write a program that finds out the smallest element after removing some integers from the natural numbers.

We have given an array of elements and k value. Remove all the elements from natural numbers that are present in the given array. And then find the k-th smallest number from the remaining natural numbers.

Let's see the steps to solve the problem.

  • Initialise the array and k.
  • Initialise an array and initialise all the elements with 0 except the elements present in the given array.
  • Write a loop that iterates till the size of the given array.
    • Decrement the value of k if the current element is not present in the above array.
    • Return the current value when k becomes zero.
  • Return 0.


Let's see the code.

 Live Demo

#include <bits/stdc++.h>
#define MAX 1000000
using namespace std;
int smallestNumber(int arr[], int n, int k) {
   int flag[MAX];
   memset(flag, 0, sizeof flag);
   for (int i = 0; i < n; i++) {
      flag[arr[i]] = 1;
   for (int i = 1; i < MAX; i++) {
      if (flag[i] != 1) {
      if (!k) {
         return i;
   return 0;
int main() {
   int k = 2;
   int arr[] = { 3, 5 };
   cout << smallestNumber(arr, 2, k) << endl;
   return 0;


If you run the above code, then you will get the following result.



If you have any queries in the tutorial, mention them in the comment section.

Updated on: 09-Apr-2021


Kickstart Your Career

Get certified by completing the course

Get Started