Find the difference of count of equal elements on the right and the left for each element

Last Updated : 11 Aug, 2021

Given an array arr[] of size N. The task is to find X – Y for each of the element where X is the count of j such that arr[i] = arr[j] and j > i. Y is the count of j such that arr[i] = arr[j] and j < i.
Examples:

Input: arr[] = {1, 2, 3, 2, 1}
Output: 1 1 0 -1 -1
For index 0, X – Y = 1 – 0 = 1
For index 1, X – Y = 1 – 0 = 1
For index 2, X – Y = 0 – 0 = 0
For index 3, X – Y = 0 – 1 = -1
For index 4, X – Y = 0 – 1 = -1
Input: arr[] = {1, 1, 1, 1, 1}
Output: 4 2 0 -2 -4

Approach: An efficient approach is to use a map. One map is to store the count of each element in the array and another map to count the number of same elements left to each element.
Below is the implementation of the above approach:

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the count of equal
// elements to the right - count of equal
// elements to the left for each of the element
void right_left(int a[], int n)
{
 
    // Maps to store the frequency and same
    // elements to the left of an element
    unordered_map<int, int> total, left;
 
    // Count the frequency of each element
    for (int i = 0; i < n; i++)
        total[a[i]]++;
 
    for (int i = 0; i < n; i++) {
 
        // Print the answer for each element
        cout << (total[a[i]] - 1 - (2 * left[a[i]])) << " ";
 
        // Increment it's left frequency
        left[a[i]]++;
    }
}
 
// Driver code
int main()
{
    int a[] = { 1, 2, 3, 2, 1 };
    int n = sizeof(a) / sizeof(a[0]);
 
    right_left(a, n);
 
    return 0;
}

Java

// Java implementation of the approach
import java.util.*;
 
class GFG
{
 
// Function to find the count of equal
// elements to the right - count of equal
// elements to the left for each of the element
static void right_left(int a[], int n)
{
 
    // Maps to store the frequency and same
    // elements to the left of an element
    Map<Integer, Integer> total = new HashMap<>();
    Map<Integer, Integer> left = new HashMap<>();
 
    // Count the frequency of each element
    for (int i = 0; i < n; i++)
        total.put(a[i],
        total.get(a[i]) == null ? 1 :
        total.get(a[i]) + 1);
 
    for (int i = 0; i < n; i++)
    {
 
        // Print the answer for each element
        System.out.print((total.get(a[i]) - 1 -
                         (2 * (left.containsKey(a[i]) == true ?
                               left.get(a[i]) : 0))) + " ");
 
        // Increment it's left frequency
        left.put(a[i],
        left.get(a[i]) == null ? 1 :
        left.get(a[i]) + 1);
    }
}
 
// Driver code
public static void main(String[] args)
{
    int a[] = { 1, 2, 3, 2, 1 };
    int n = a.length;
 
    right_left(a, n);
}
}
 
// This code is contributed by Princi Singh

Python3

# Python3 implementation of the approach
 
# Function to find the count of equal
# elements to the right - count of equal
# elements to the left for each of the element
def right_left(a, n) :
 
    # Maps to store the frequency and same
    # elements to the left of an element
    total = dict.fromkeys(a, 0);
    left = dict.fromkeys(a, 0);
 
    # Count the frequency of each element
    for i in range(n) :
        if a[i] not in total :
            total[a[i]] = 1
        total[a[i]] += 1;
 
    for i in range(n) :
 
        # Print the answer for each element
        print(total[a[i]] - 1 - (2 * left[a[i]]),
                                      end = " ");
 
        # Increment it's left frequency
        left[a[i]] += 1;
 
# Driver code
if __name__ == "__main__" :
 
    a = [ 1, 2, 3, 2, 1 ];
    n = len(a);
 
    right_left(a, n);
 
# This code is contributed by AnkitRai01

C#

// C# implementation of the approach
using System;
using System.Collections.Generic;
     
class GFG
{
 
// Function to find the count of equal
// elements to the right - count of equal
// elements to the left for each of the element
static void right_left(int []a, int n)
{
 
    // Maps to store the frequency and same
    // elements to the left of an element
    Dictionary<int, int> total = new Dictionary<int, int>();
    Dictionary<int, int> left = new Dictionary<int, int>();
 
    // Count the frequency of each element
    for (int i = 0; i < n; i++)
    {
        if(total.ContainsKey(a[i]))
        {
            total[a[i]] = total[a[i]] + 1;
        }
        else{
            total.Add(a[i], 1);
        }
    }
 
    for (int i = 0; i < n; i++)
    {
 
        // Print the answer for each element
        Console.Write((total[a[i]] - 1 -
                      (2 * (left.ContainsKey(a[i]) == true ?
                                   left[a[i]] : 0))) + " ");
 
        // Increment it's left frequency
        if(left.ContainsKey(a[i]))
        {
            left[a[i]] = left[a[i]] + 1;
        }
        else
        {
            left.Add(a[i], 1);
        }
    }
}
 
// Driver code
public static void Main(String[] args)
{
    int []a = { 1, 2, 3, 2, 1 };
    int n = a.Length;
 
    right_left(a, n);
}
}
 
// This code is contributed by Rajput-Ji

Javascript

<script>
 
    // Javascript implementation of the approach
 
// Function to find the count of equal
// elements to the right - count of equal
// elements to the left for each of the element
function right_left(a, n)
{
   
    // Maps to store the frequency and same
    // elements to the left of an element
   let total = new Map();
    let left = new Map();
   
    // Count the frequency of each element
    for (let i = 0; i < n; i++)
        total.set(a[i],
        total.get(a[i]) == null ? 1 :
        total.get(a[i]) + 1);
   
    for (let i = 0; i < n; i++)
    {
   
        // Prlet the answer for each element
        document.write((total.get(a[i]) - 1 -
                         (2 * (left.has(a[i]) == true ?
                               left.get(a[i]) : 0))) + " ");
   
        // Increment it's left frequency
        left.set(a[i],
        left.get(a[i]) == null ? 1 :
        left.get(a[i]) + 1);
    }
}
     
    // Driver code
     
        let a = [ 1, 2, 3, 2, 1 ];
    let n = a.length;
   
    right_left(a, n);
     
    // This code is contributed by susmitakundugoaldanga.
</script>

Output:

1 1 0 -1 -1

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