Find a number K such that Array contains at least K numbers greater than or equal to K

Given an array arr[] of non-negative integers of size N, the task is to find an integer H such that at least K integers in the array are greater or equal to K.

Examples:

Input: arr[] = [3, 0, 6, 1, 5]
Output: 3
Explanation: There are 3 number greater than or equal to 3 in the array i.e. 3, 6 and 5.

Input: arr[] = [9, 10, 7, 5, 0, 10, 2, 0]
Output: 5

Approach: Using Hashing
The number integer K can not be greater than the size of arr[]. So, maintain the frequency of each element in a frequency array(hash table). Then traverse the frequency array from the end and return the first index that matches the condition.



Below is the implementation of the above approach.

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// C++ code for the above program
  
#include <bits/stdc++.h>
using namespace std;
  
int getNumberK(vector<int>& S)
{
    // vector to store freq.
    vector<int> freq(S.size() + 1,
                     0);
  
    // Filling freq vector.
    for (int i = 0; i < S.size();
         i++) {
  
        if (S[i] < S.size())
            freq[S[i]]++;
        else
            freq[S.size()]++;
    }
  
    int total = 0;
  
    // Finding K number.
    for (int i = S.size(); i >= 0;
         i--) {
  
        total += freq[i];
        if (total >= i)
            return i;
    }
  
    // No K number found.
    return 0;
}
  
// Driver code
int main()
{
    vector<int> arr{ 3, 0, 6, 1, 5 };
    cout << getNumberK(arr) << '\n';
    return 0;
}

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Output:

3


Time Complexity O(N)
Space Complexity O(N)

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