Find first k natural numbers missing in given array

Given an array of size n and a number k, we need to print first k natural numbers that are not there in given array.

Examples:

Input : [2 3 4] 
         k = 3
Output : [1 5 6]

Input  : [-2 -3 4] 
          k = 2
Output : [1 2]

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

1) Sort the given array.
2) After sorting, we find position of first positive number in array.
3) Now we traverse the array and keep printing elements in gaps between two consecutive array elements.
4) If gaps don’t cover k missing numbers, we print numbers greater than the largest array element.

C++

// C++ program to find missing k numbers 
// in an array. 
#include <bits/stdc++.h> 
using namespace std; 
  
// Prints first k natural numbers in 
// arr[0..n-1] 
void printKMissing(int arr[], int n, int k) 
{ 
    sort(arr, arr + n); 
  
    // Find first positive number 
    int i = 0; 
    while (i < n && arr[i] <= 0) 
        i++; 
  
    // Now find missing numbers 
    // between array elements 
    int count = 0, curr = 1; 
    while (count < k && i < n) { 
        if (arr[i] != curr) { 
            cout << curr << " "; 
            count++; 
        } 
        else
            i++; 
  
        curr++; 
    } 
  
    // Find missing numbers after 
    // maximum. 
    while (count < k) { 
        cout << curr << " "; 
        curr++; 
        count++; 
    } 
} 
  
// Driver code 
int main() 
{ 
    int arr[] = { 2, 3, 4 }; 
    int n = sizeof(arr) / sizeof(arr[0]); 
    int k = 3; 
    printKMissing(arr, n, k); 
    return 0; 
}

Java

// Java program to find missing k numbers 
// in an array. 
import java.util.Arrays; 
  
class GFG { 
    // Prints first k natural numbers in 
    // arr[0..n-1] 
    static void printKMissing(int[] arr, int n, int k) 
    { 
        Arrays.sort(arr); 
  
        // Find first positive number 
        int i = 0; 
        while (i < n && arr[i] <= 0) 
            i++; 
  
        // Now find missing numbers 
        // between array elements 
        int count = 0, curr = 1; 
        while (count < k && i < n) { 
            if (arr[i] != curr) { 
                System.out.print(curr + " "); 
                count++; 
            } 
            else
                i++; 
            curr++; 
        } 
  
        // Find missing numbers after 
        // maximum. 
        while (count < k) { 
            System.out.print(curr + " "); 
            curr++; 
            count++; 
        } 
    } 
  
    // Driver code 
    public static void main(String[] args) 
    { 
        int[] arr = { 2, 3, 4 }; 
        int n = arr.length; 
        int k = 3; 
        printKMissing(arr, n, k); 
    } 
} 
/* This code is contributed by Mr. Somesh Awasthi */

Python3

# Python3 program to find missing  
# k numbers in an array.  
  
# Prints first k natural numbers  
# in arr[0..n-1]  
def printKMissing(arr, n, k) : 
      
    arr.sort()  
  
    # Find first positive number  
    i = 0
    while (i < n and arr[i] <= 0) : 
        i = i + 1
  
    # Now find missing numbers  
    # between array elements  
    count = 0
    curr = 1
    while (count < k and i < n) :  
        if (arr[i] != curr) : 
            print(str(curr) + " ", end = '') 
            count = count + 1
        else: 
            i = i + 1
          
        curr = curr + 1
          
    # Find missing numbers after  
    # maximum.  
    while (count < k) : 
        print(str(curr) + " ", end = '')  
        curr = curr + 1
        count = count + 1
          
# Driver code  
arr = [ 2, 3, 4 ]  
n = len(arr)  
k = 3
printKMissing(arr, n, k);  
  
# This code is contributed  
# by Yatin Gupta 

C#

// C# program to find missing  
// k numbers in an array. 
using System; 
  
class GFG { 
    // Prints first k natural numbers 
    // in arr[0..n-1] 
    static void printKMissing(int[] arr,  
                              int n,  
                              int k) 
    { 
        Array.Sort(arr); 
  
        // Find first positive number 
        int i = 0; 
        while (i < n && arr[i] <= 0) 
            i++; 
  
        // Now find missing numbers 
        // between array elements 
        int count = 0, curr = 1; 
        while (count < k && i < n) { 
            if (arr[i] != curr) { 
                Console.Write(curr + " "); 
                count++; 
            } 
            else
                i++; 
            curr++; 
        } 
  
        // Find missing numbers  
        // after maximum. 
        while (count < k) { 
            Console.Write(curr + " "); 
            curr++; 
            count++; 
        } 
    } 
  
    // Driver code 
    public static void Main() 
    { 
        int[] arr = {2, 3, 4}; 
        int n = arr.Length; 
        int k = 3; 
        printKMissing(arr, n, k); 
    } 
} 
  
// This code is contributed by Nitin Mittal 

PHP

<?php 
// PHP program to find missing k numbers 
// in an array. 
  
// Prints first k natural numbers in 
// arr[0..n-1] 
function printKMissing($arr, $n, $k) 
{ 
    sort($arr); sort($arr , $n); 
  
    // Find first positive number 
    $i = 0; 
    while ($i < $n && $arr[$i] <= 0) 
        $i++; 
  
    // Now find missing numbers 
    // between array elements 
    $count = 0; $curr = 1; 
    while ($count < $k && $i < $n) { 
        if ($arr[$i] != $curr) { 
            echo $curr , " "; 
            $count++; 
        } 
        else
            $i++; 
  
        $curr++; 
    } 
  
    // Find missing numbers after 
    // maximum. 
    while ($count < $k) { 
        echo $curr , " "; 
        $curr++; 
        $count++; 
    } 
} 
  
    // Driver code 
    $arr =array ( 2, 3, 4 ); 
    $n = sizeof($arr); 
    $k = 3; 
    printKMissing($arr, $n, $k); 
  
// This code is contributed by Nitin Mittal. 
?> 

Output:

1 5 6

Time Complexity: O(n Log n)

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My Personal Notes

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

C++

Java

Python3

C#

PHP

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