# 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

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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.

 `// C++ code for the above program ` ` `  `#include ` `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; ` `} `

Output:

```3
```

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

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