k-th missing element in an unsorted array

Given an unsorted sequence a[], the task is to find the K-th missing contiguous element in the increasing sequence of the array elements i.e. consider the array in sorted order and find the kth missing number. If no k-th missing element is there output -1.

Note: Only elements exists in the range of minimum and maximum element to be considered.
Examples:

Input: arr[] = {2, 4, 10, 7}, k = 5
Output: 9
Missing elements in the given array: 3, 5, 6, 8, 9
5th missing is 9.

Input: arr[] = {1, 3, 4}, k = 5
Output: -1

Method-1: Sort the array and use the approach used in the k-th missing element in a sorted array.

Method-2:

  1. Insert all the elements in an unordered_set.
  2. Find the minimum and maximum element of the array.
  3. Traverse the elements from minimum to maximum.



    • Check if current element is present in the set or not.
    • If not then check if this is kth missing by counting the missing elements.
    • Return the current element if this is current missing.

Below is the implementation of the above approach:

C++

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// C++ implementation of the above approach
#include <bits/stdc++.h>
using namespace std;
  
// Function to find the sum
// of minimum of all subarrays
int findKth(int arr[], int n, int k)
{
  
    unordered_set<int> missing;
    int count = 0;
  
    // Insert all the elements in a set
    for (int i = 0; i < n; i++)
        missing.insert(arr[i]);
  
    // Find the maximum and minimum element
    int maxm = *max_element(arr, arr + n);
    int minm = *min_element(arr, arr + n);
  
    // Traverse from the minimum to maximum element
    for (int i = minm + 1; i < maxm; i++) {
        // Check if "i" is missing
        if (missing.find(i) == missing.end())
            count++;
  
        // Check if it is kth missing
        if (count == k)
            return i;
    }
  
    // If no kth element is missing
    return -1;
}
  
// Driver code
int main()
{
    int arr[] = { 2, 10, 9, 4 };
    int n = sizeof(arr) / sizeof(arr[0]);
    int k = 5;
    cout << findKth(arr, n, k);
  
    return 0;
}

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Java














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// Java implementation of the above approach 


import java.util.*; 


  


class GFG 


{ 


  


    // Function to find the sum 


    // of minimum of all subarrays 


    static int findKth(int arr[], int n, int k)  


    { 


  


        HashSet<Integer> missing = new HashSet<>(); 


        int count = 0; 


  


        // Insert all the elements in a set 


        for (int i = 0; i < n; i++) 


        { 


            missing.add(arr[i]); 


        } 


  


        // Find the maximum and minimum element 


        int maxm = Arrays.stream(arr).max().getAsInt(); 


        int minm = Arrays.stream(arr).min().getAsInt(); 


  


        // Traverse from the minimum to maximum element 


        for (int i = minm+1; i < maxm; i++) 


        { 


            // Check if "i" is missing 


            if (!missing.contains(i))  


            {  


                count++; 


            } 


  


            // Check if it is kth missing 


            if (count == k) 


            {  


                return i; 


            } 


        } 


          


        // If no kth element is missing 


        return -1; 


    } 


  


    // Driver code 


    public static void main(String[] args)  


    { 


        int arr[] = {2, 10, 9, 4}; 


        int n = arr.length; 


        int k = 5; 


        System.out.println(findKth(arr, n, k)); 


    } 


} 


  


/* This code contributed by PrinciRaj1992 */



















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Python














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# Python3 implementation of the above approach 


  


# Function to find the sum 


# of minimum of all subarrays 


def findKth( arr, n, k): 


  


    missing = dict() 


    count = 0


  


    # Insert all the elements in a set 


    for i in range(n): 


        missing[arr[i]] = 1


  


    # Find the maximum and minimum element 


    maxm = max(arr) 


    minm = min(arr) 


  


    # Traverse from the minimum to maximum element 


    for i in range(minm + 1, maxm): 


          


        # Check if "i" is missing 


        if (i not in missing.keys()): 


            count += 1


  


        # Check if it is kth missing 


        if (count == k): 


            return i 


      


    # If no kth element is missing 


    return -1


  


# Driver code 


arr = [2, 10, 9, 4 ] 


n = len(arr) 


k = 5


print(findKth(arr, n, k)) 


  


# This code is contributed by Mohit Kumar 



















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C#














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// C# implementation of the above approach  


using System; 


using System.Linq; 


using System.Collections.Generic; 


  


class GFG  


{  


  


    // Function to find the sum  


    // of minimum of all subarrays  


    static int findKth(int []arr, int n, int k)  


    {  


  


        HashSet<int> missing = new HashSet<int>();  


        int count = 0;  


  


        // Insert all the elements in a set  


        for (int i = 0; i < n; i++)  


        {  


            missing.Add(arr[i]);  


        }  


  


        // Find the maximum and minimum element  


        int maxm = arr.Max();  


        int minm = arr.Min();  


  


        // Traverse from the minimum to maximum element  


        for (int i = minm + 1; i < maxm; i++)  


        {  


            // Check if "i" is missing  


            if (!missing.Contains(i))  


            {  


                count++;  


            }  


  


            // Check if it is kth missing  


            if (count == k)  


            {  


                return i;  


            }  


        }  


          


        // If no kth element is missing  


        return -1;  


    }  


  


    // Driver code  


    public static void Main(String[] args)  


    {  


        int []arr = {2, 10, 9, 4};  


        int n = arr.Length;  


        int k = 5;  


        Console.WriteLine(findKth(arr, n, k));  


    }  


}  


  


// This code has been contributed by 29AjayKumar 



















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PHP














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<?php 


// PHP implementation of the above approach  


  


// Function to find the sum  


// of minimum of all subarrays  


function findKth($arr, $n, $k) 


{ 


  


    $missing = array(); 


    $count = 0; 


  


    // Insert all the elements in a set  


    for ($i = 0; $i < $n; $i++) 


        array_push($missing, $arr[$i]); 


  


    $missing = array_unique($missing); 


      


    // Find the maximum and minimum element  


    $maxm = max($arr); 


    $minm = min($arr); 


  


    // Traverse from the minimum to 


    // maximum element  


    for ($i = $minm + 1; $i < $maxm; $i++) 


    { 


        // Check if "i" is missing  


        if (!in_array($i, $missing, false)) 


            $count += 1; 


  


        // Check if it is kth missing  


        if ($count == $k)  


            return $i; 


    } 


      


    // If no kth element is missing  


    return -1; 


} 


  


// Driver code  


$arr = array(2, 10, 9, 4);  


$n = sizeof($arr);  


$k = 5; 


  


echo findKth($arr, $n, $k); 


  


// This code is contributed by Ryuga 


?> 



















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Output:


8







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