Count Pairs with equal elements and Divisible Index Sum

Last Updated : 23 Jul, 2025

Given an array arr[] of length N and an integer k. You have to return the count of all the pairs (i, j) such that:

arr[i] == arr[j]
i<j
(i+j) is divisible by k

Examples:

Input: N = 5, k = 3, arr[] = {1, 2, 3, 2, 1}
Output: 2
Explanation:

arr[2] = arr[4] and 2<4 , (2+4) = 6 is divisible by 3.
arr[1] = arr[5] and 1<5 , (1+5) = 6 is divisible by 3.
Input: N = 6, k = 4, arr[] = {1, 1, 1, 1, 1, 1}
Output: 3
Explanation:

arr[1] = arr[3] and 1<3, (1+3) = 4 is divisible by 4.
arr[2] = arr[6] and 2<6, (2+6) = 8 is divisible by 4.
arr[3] = arr[5] and 3<5, (3+5) = 8 is divisible by 4.

Approach: This can be solved with the following idea:

Using Map data structure, Create a a list of indexes. For each value, find number of indexes divisible by k and the count of the indexes in ans.

Below are the steps involved:

Create a map containing:
- Key as a integer.
- Value as a vector of integers.
Iterate over array, insert values in map.
Iterate over map:
- For each value, Iterate over vector containing indexes:
  - Count total number of pairs possible whose sum is divisible by k.
  - Add the count to ans.
Return ans.

Below is the implementation of the code:

C++

// C++ code for the above approach:
#include <bits/stdc++.h>
#include <iostream>
using namespace std;

// Function to calculate number of pairs
int CountPairs(int N, int k, vector<int>& v)
{

    unordered_map<int, vector<int> > M;

    // Iterate over the array, store
    // their indexes
    for (int i = 0; i < N; i++) {

        M[v[i]].push_back((i + 1) % k);
    }

    long long ans = 0;
    for (auto x : M) {

        // Iterate over the vector
        auto& y = x.second;
        unordered_map<int, int> mod;
        for (int i : y) {

            // Check how may indexes possible
            int need = (k - i) % k;

            // Add it to ans
            ans += mod[need];

            mod[i]++;
        }
    }

    // Return total number of pairs
    return (int)ans;
}

// Driver code
int main()
{

    int N = 5;
    int k = 3;
    vector<int> arr = { 1, 2, 3, 2, 1 };

    // Function call
    cout << CountPairs(N, k, arr);

    return 0;
}

Java

import java.util.*;

public class Main {
    // Function to calculate number of pairs
    static int countPairs(int N, int k, int[] v) {
        Map<Integer, List<Integer>> M = new HashMap<>();

        // Iterate over the array, store their indexes
        for (int i = 0; i < N; i++) {
            M.computeIfAbsent(v[i], key -> new ArrayList<>()).add((i + 1) % k);
        }

        long ans = 0;
        for (Map.Entry<Integer, List<Integer>> entry : M.entrySet()) {
            List<Integer> y = entry.getValue();
            Map<Integer, Integer> mod = new HashMap<>();
            for (int i : y) {
                // Check how many indexes are possible
                int need = (k - i) % k;

                // Add it to ans
                ans += mod.getOrDefault(need, 0);

                mod.put(i, mod.getOrDefault(i, 0) + 1);
            }
        }

        // Return total number of pairs
        return (int) ans;
    }

    // Driver code
    public static void main(String[] args) {
        int N = 5;
        int k = 3;
        int[] arr = {1, 2, 3, 2, 1};

        // Function call
        System.out.println(countPairs(N, k, arr));
    }
}

Python3

from collections import defaultdict

# Function to calculate number of pairs
def count_pairs(N, k, v):
    M = defaultdict(list)

    # Iterate over the array, store their indexes
    for i in range(N):
        M[v[i]].append((i + 1) % k)

    ans = 0
    for x in M.items():

        # Iterate over the vector
        y = x[1]
        mod = defaultdict(int)
        for i in y:

            # Check how many indexes possible
            need = (k - i) % k

            # Add it to ans
            ans += mod[need]

            mod[i] += 1

    # Return total number of pairs
    return int(ans)

# Driver code
N = 5
k = 3
arr = [1, 2, 3, 2, 1]

# Function call
print(count_pairs(N, k, arr))

using System;
using System.Collections.Generic;

class GFG
{
    // Function to calculate the number of the pairs
    static int CountPairs(int N, int k, List<int> v)
    {
        Dictionary<int, List<int>> M = new Dictionary<int, List<int>>();

        // Iterate over the array
      // store their indexes
        for (int i = 0; i < N; i++)
        {
            int val = v[i];
            if (!M.ContainsKey(val))
            {
                M[val] = new List<int>();
            }
            M[val].Add((i + 1) % k);
        }
        long ans = 0;
        foreach (var pair in M)
        {
            var y = pair.Value;
            Dictionary<int, int> mod = new Dictionary<int, int>();

            foreach (int i in y)
            {
                // Check how many indexes are possible
                int need = (k - i) % k;
                // Add it to ans
                if (mod.ContainsKey(need))
                {
                    ans += mod[need];
                }
                if (!mod.ContainsKey(i))
                {
                    mod[i] = 0;
                }
                mod[i]++;
            }
        }

        // Return the total number of the pairs
        return (int)ans;
    }
    // Driver code
    static void Main()
    {
        int N = 5;
        int k = 3;
        List<int> arr = new List<int> { 1, 2, 3, 2, 1 };
        // Function call
        Console.WriteLine(CountPairs(N, k, arr));
    }
}

JavaScript

// JavaScript code for the above approach

// Function to calculate number of pairs
function countPairs(N, k, v) {
    const M = new Map();

    // Iterate over the array, store their indexes
    for (let i = 0; i < N; i++) {
        const index = (i + 1) % k;
        if (M.has(v[i])) {
            M.get(v[i]).push(index);
        } 
        else {
            M.set(v[i], [index]);
        }
    }

    let ans = 0;
    for (const [key, value] of M) {
        const y = value;
        const mod = new Map();
        for (let i = 0; i < y.length; i++) {
            // Check how many indexes are possible
            const need = (k - y[i]) % k;
            // Add it to ans
            ans += mod.get(need) || 0;
            mod.set(y[i], (mod.get(y[i]) || 0) + 1);
        }
    }

    // Return total number of pairs
    return ans;
}

// Inputs
const N = 5;
const k = 3;
const arr = [1, 2, 3, 2, 1];

// Function call
console.log(countPairs(N, k, arr));

Output

Time Complexity: O(N*log N)
Auxiliary Space: O(N)

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Count Pairs with equal elements and Divisible Index Sum

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