Fill an array based on frequency where elements are in range from 0 to n-1

Given an array of positive integers with duplicate allowed. The array contains elements from range 0 to n-1. The task is to fill the array such that arr[i] contains the frequency of i.

Examples :  

Input  : arr[] = {1, 4, 3, 4, 1, 1, 4, 4, 4, 7}
Output : arr[] = {0, 3, 0, 1, 5, 0, 0, 1, 0, 0}
Here 0 appears 0 times, so arr[0] is 0
     1 appears 3 times 
     2 appears 0 times 
     3 appears 0 times 
     4 appears 5 times 
     ..........

Input  : arr[] = {1, 2, 3, 4, 1, 1, 4, 5, 6, 7}
Output : arr[] = {0, 3, 1, 1, 2, 1, 1, 1, 0, 0} 

A simple solution of this problem is to run two loops. The outer loop picks elements one by one. The inner loop counts frequency of the picked element and stores frequency in final array.

An efficient solution is to use an auxiliary array of size n.  

1) Create an array temp[0..n-1] and 
   initialize it as 0.
2) Traverse given array and do following
   for every element arr[i].
     temp[arr[i]]++
3) Copy temp[] to arr[].

Below is the implementation of above steps. 

// C++ program to fill an array with frequencies.
#include<bits/stdc++.h>
using namespace std;

// Fills arr[] with frequencies of elements
void fillWithFreq(int arr[], int n)
{
    int temp[n];
    memset(temp, 0, sizeof(temp));

    // Fill temp with frequencies
    for (int i=0; i<n; i++)
        temp[arr[i]] += 1;

    // Copy temp to array
    for (int i=0; i<n; i++)
        arr[i] = temp[i];
}

// Driver code
int main()
{
    int arr[] = {5, 2, 3, 4, 5, 5, 4, 5, 6, 7};
    int n = sizeof(arr)/sizeof(arr[0]);
    fillWithFreq(arr, n);
    for (int i=0; i<n; i++)
        cout << arr[i] << " ";
    return 0;
}

// Java program to fill an array with frequencies.
import java.util.Arrays;

class GFG {
    
    // Fills arr[] with frequencies of elements
    static void fillWithFreq(int arr[], int n)
    {
        
        int temp[]=new int[n];
        Arrays.fill(temp, 0);
    
        // Fill temp with frequencies
        for (int i = 0; i < n; i++)
            temp[arr[i]] += 1;
    
        // Copy temp to array
        for (int i = 0; i < n; i++)
            arr[i] = temp[i];
    }
    
    // Driver method
    public static void main(String[] args)
    {
        
        int arr[] = {5, 2, 3, 4, 5, 5, 4, 5, 6, 7};
        int n = arr.length;
        
        fillWithFreq(arr, n);
        
        for (int i=0; i<n; i++)
            System.out.print(arr[i] + " ");
    }
}

// This code is contributed by Anant Agarwal.

# Python3 program to fill an
# array with frequencies.

# Fills arr[] with frequencies of elements
def fillWithFreq(arr, n):

    temp = [0 for i in range(n)]

    # Fill temp with frequencies
    for i in range(n):
        temp[arr[i]] += 1

    # Copy temp to array
    for i in range(n):
        arr[i] = temp[i]

# Driver Code
arr = [5, 2, 3, 4, 5, 5, 4, 5, 6, 7]
n = len(arr)
fillWithFreq(arr, n)
for i in range(n):
    print(arr[i], end = " ")
    
# This code is contributed by Anant Agarwal.

// C# program to fill an 
// array with frequencies.
using System;

class GFG {
    
    // Fills arr[] with frequencies of elements
    static void fillWithFreq(int []arr, int n)
    {
        int []temp = new int[n];
        for(int i = 0; i < n; i++)
         temp[i] = 0;
         
        // Fill temp with frequencies
        for (int i = 0; i < n; i++)
            temp[arr[i]] += 1;
    
        // Copy temp to array
        for (int i = 0; i < n; i++)
            arr[i] = temp[i];
    }
    
    // Driver method
    public static void Main()
    {
        int []arr = {5, 2, 3, 4, 5,
                     5, 4, 5, 6, 7};
        int n = arr.Length;
        
        // Function calling
        fillWithFreq(arr, n);
        
        for (int i = 0; i < n; i++)
        Console.Write(arr[i] + " ");
    }
}

// This code is contributed by nitin mittal.

<?php
// PHP program to fill an 
// array with frequencies.

// Fills arr[] with frequencies
// of elements
function fillWithFreq($arr, $n)
{
    $temp = array();
    for($i = 0; $i < $n; $i++)
    $temp[$i] = 0;
    
    // Fill temp with frequencies
    for ($i = 0; $i < $n; $i++)
        $temp[$arr[$i]] += 1;

    // Copy temp to array
    for ($i = 0; $i < $n; $i++)
        $arr[$i] = $temp[$i];
        
    // print array
    for ($i = 0; $i < $n; $i++)
    echo $arr[$i] . " ";
}

// Driver method
$arr = array(5, 2, 3, 4, 5,
             5, 4, 5, 6, 7);
$n = count($arr);

// Function calling
fillWithFreq($arr, $n);

// This code is contributed by Sam007
?>

<script>
    // Javascript program to fill an 
    // array with frequencies.
    
    // Fills arr[] with frequencies of elements
    function fillWithFreq(arr, n)
    {
        let temp = new Array(n);
        for(let i = 0; i < n; i++)
         temp[i] = 0;
           
        // Fill temp with frequencies
        for (let i = 0; i < n; i++)
            temp[arr[i]] += 1;
      
        // Copy temp to array
        for (let i = 0; i < n; i++)
            arr[i] = temp[i];
    }
    
    let arr = [5, 2, 3, 4, 5, 5, 4, 5, 6, 7];
    let n = arr.length;

    // Function calling
    fillWithFreq(arr, n);

    for (let i = 0; i < n; i++)
      document.write(arr[i] + " ");
    
</script>

0 0 1 1 2 4 1 1 0 0 

Time Complexity : O(n) 
Auxiliary Space : O(n)