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Number of pairs such that their HCF and LCM is equal
  • Last Updated : 04 May, 2020

Given an array a[] of non-negative integers. The task is to count the number of pairs (i, j) in the array such that LCM(a[i], a[j]) = HCF(a[i], a[j]).

Note: The pair (i, j) and (j, i) are considered same and i should not be equal to j.

Examples:

Input : a[] = {3, 4, 3, 4, 5}
Output : 2
Pairs are (3, 3) and (4, 4)

Input  : a[] = {1, 1, 1}
Output : 3

Naive approach: Generate all possible pairs and count pairs with equal HCF and LCM.

C++



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// Naive C++ program to count number of pairs
// such that their hcf and lcm are equal
#include <bits/stdc++.h>
using namespace std;
  
// Function to return HCF of two numbers
int gcd(int a, int b)
{
    if (a == 0)
        return b;
    return gcd(b % a, a);
}
  
// Function to return LCM of two numbers
int lcm(int a, int b)
{
    return (a * b) / gcd(a, b);
}
  
// Returns the number of valid pairs
int countPairs(int arr[], int n)
{
    int ans = 0; // initializing answer
  
    // Traversing the array. For each array
    // element, checking if it
    // follow the condition
    for (int i = 0; i < n; i++)
        for (int j = i + 1; j < n; j++)
            if (lcm(arr[i], arr[j]) == gcd(arr[i], arr[j]))
                ans++;
    return ans;
}
  
// Driver function
int main()
{
    int arr[] = { 1, 1, 1 };
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << countPairs(arr, n);
    return 0;
}

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Java

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// Naive Java program to count number of pairs
// such that their hcf and lcm are equal
import java.util.*;
  
class GFG{
   
// Function to return HCF of two numbers
static int gcd(int a, int b)
{
    if (a == 0)
        return b;
    return gcd(b % a, a);
}
   
// Function to return LCM of two numbers
static int lcm(int a, int b)
{
    return (a * b) / gcd(a, b);
}
   
// Returns the number of valid pairs
static int countPairs(int arr[], int n)
{
    int ans = 0; // initializing answer
   
    // Traversing the array. For each array
    // element, checking if it
    // follow the condition
    for (int i = 0; i < n; i++)
        for (int j = i + 1; j < n; j++)
            if (lcm(arr[i], arr[j]) == gcd(arr[i], arr[j]))
                ans++;
    return ans;
}
   
// Driver function
public static void main(String[] args)
{
    int arr[] = { 1, 1, 1 };
    int n = arr.length;
    System.out.print(countPairs(arr, n));
}
}
  
// This code is contributed by Rajput-Ji

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Python3

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# Naive Python program to count number of pairs
# such that their hcf and lcm are equal
  
# Function to return HCF of two numbers
def gcd(a, b):
    if (a == 0):
        return b;
    return gcd(b % a, a);
  
# Function to return LCM of two numbers
def lcm(a, b):
    return (a * b) / gcd(a, b);
  
# Returns the number of valid pairs
def countPairs(arr, n):
    ans = 0; # initializing answer
  
    # Traversing the array. For each array
    # element, checking if it
    # follow the condition
    for i in range(n):
        for j in range(i+1,n):
            if (lcm(arr[i], arr[j]) == gcd(arr[i], arr[j])):
                ans+=1;
    return ans;
  
# Driver function
if __name__ == '__main__':
    arr = [ 1, 1, 1 ];
    n = len(arr);
    print(countPairs(arr, n));
  
# This code is contributed by 29AjayKumar

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C#

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// Naive C# program to count number of pairs
// such that their hcf and lcm are equal
using System;
  
class GFG{
    
// Function to return HCF of two numbers
static int gcd(int a, int b)
{
    if (a == 0)
        return b;
    return gcd(b % a, a);
}
    
// Function to return LCM of two numbers
static int lcm(int a, int b)
{
    return (a * b) / gcd(a, b);
}
    
// Returns the number of valid pairs
static int countPairs(int []arr, int n)
{
    int ans = 0; // initializing answer
    
    // Traversing the array. For each array
    // element, checking if it
    // follow the condition
    for (int i = 0; i < n; i++)
        for (int j = i + 1; j < n; j++)
            if (lcm(arr[i], arr[j]) == gcd(arr[i], arr[j]))
                ans++;
    return ans;
}
    
// Driver function
public static void Main(String[] args)
{
    int []arr = { 1, 1, 1 };
    int n = arr.Length;
    Console.Write(countPairs(arr, n));
}
}
  
// This code is contributed by 29AjayKumar

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Output:

3

Efficient Approach: If we observe carefully, it can be proved that the HCF and LCM of two numbers can be equal only when the numbers are also equal.

Proof:

Let,
HCF(a[i], a[j]) = LCM(a[i], a[j]) = K

Since HCF(a[i], a[j]) = k,
a[i] = k*n1, a[j] = k*n2, for some natural numbers n1, n2

We know that,
HCF × LCM = Product of the two numbers

Therefore,
k*k = k*n1 × k*n2
or, n1*n2 = 1

Implies, n1 = n2 = 1, since n1, n2 are natural numbers.

Therefore,
a[i] = k*n1 = k, a[j] = k*n2 = k
i.e. the numbers must be equal.

So we have to count pairs wih same elements in the pair.

It is observed that only the pairs of the form (arr[i], arr[j]) where arr[i] = arr[j] will satisfy the given condition. So, the problem now gets reduced to finding the number of pairs (arr[i], arr[j]) such that arr[i] = arr[j].

Below is the implementation of the above approach:

C++

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// C++ program to count number of pairs
// such that their hcf and lcm are equal
#include <bits/stdc++.h>
using namespace std;
  
// Function to count number of pairs
// such that their hcf and lcm are equal
int countPairs(int a[], int n)
{
    // Store frequencies of array elements
    unordered_map<int, int> frequency;
    for (int i = 0; i < n; i++) {
        frequency[a[i]]++;
    }
  
    int count = 0;
  
    // Count of pairs (arr[i], arr[j])
    // where arr[i] = arr[j]
    for (auto x : frequency) {
        int f = x.second;
        count += f * (f - 1) / 2;
    }
  
    // Count of pairs (arr[i], arr[j]) where
    // arr[i] = arr[j],
    return count;
}
  
// Driver function
int main()
{
    int arr[] = { 1, 1, 1 };
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << countPairs(arr, n);
    return 0;
}

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Java

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// Java program to count number of pairs
// such that their hcf and lcm are equal
  
import java.util.*;
  
class GFG{
   
// Function to count number of pairs
// such that their hcf and lcm are equal
static int countPairs(int arr[], int n)
{
    // Store frequencies of array elements
    HashMap<Integer,Integer> frequency =
            new HashMap<Integer,Integer>();
    for (int i = 0; i < n; i++) {
        if(frequency.containsKey(arr[i])){
            frequency.put(arr[i], frequency.get(arr[i])+1);
        }else{
            frequency.put(arr[i], 1);
    }
    }
   
    int count = 0;
   
    // Count of pairs (arr[i], arr[j])
    // where arr[i] = arr[j]
    for (Map.Entry<Integer,Integer> x : frequency.entrySet()) {
        int f = x.getValue();
        count += f * (f - 1) / 2;
    }
   
    // Count of pairs (arr[i], arr[j]) where
    // arr[i] = arr[j],
    return count;
}
   
// Driver function
public static void main(String[] args)
{
    int arr[] = { 1, 1, 1 };
    int n = arr.length;
    System.out.print(countPairs(arr, n));
}
}
  
// This code contributed by sapnasingh4991

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C#

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// C# program to count number of pairs
// such that their hcf and lcm are equal
using System;
using System.Collections.Generic;
  
class GFG{
  
// Function to count number of pairs
// such that their hcf and lcm are equal
static int countPairs(int []a, int n)
{
    // Store frequencies of array elements
    Dictionary<int, int> frequency = new Dictionary<int,int>();
        for (int i = 0; i < n; i++) 
        
            if (frequency.ContainsKey(a[i]))  
            
                var val = frequency[a[i]]; 
                frequency.Remove(a[i]); 
                frequency.Add(a[i], val + 1);  
            }  
            else
            
                frequency.Add(a[i], 1); 
            
        
           
    int count = 0;
  
    // Count of pairs (arr[i], arr[j])
    // where arr[i] = arr[j]
    foreach(KeyValuePair<int, int> entry in frequency) {
        int f = entry.Value;
        count += f * (f - 1) / 2;
    }
  
    // Count of pairs (arr[i], arr[j]) where
    // arr[i] = arr[j],
    return count;
}
  
// Driver function
public static void Main(String[] args)
{
    int []arr = { 1, 1, 1 };
    int n = arr.Length;
    Console.Write(countPairs(arr, n));
}
}
  
// This code is contributed by shivanisinghss2110

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Python3

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# Python 3 program to count number of pairs
# such that their hcf and lcm are equal
from collections import defaultdict
   
# Function to count number of pairs
# such that their hcf and lcm are equal
def countPairs(a, n):
  
    # Store frequencies of array elements
    frequency = defaultdict(int)
    for i in range(n) :
        frequency[a[i]] += 1
      
   
    count = 0
   
    # Count of pairs (arr[i], arr[j])
    # where arr[i] = arr[j]
    for x in frequency.keys():
        f = frequency[x]
        count += f * (f - 1) // 2
   
    # Count of pairs (arr[i], arr[j]) where
    # arr[i] = arr[j],
    return count
   
# Driver function
if __name__ == "__main__":
      
    arr = [ 1, 1, 1 ]
    n = len(arr)
    print(countPairs(arr, n))
  
# This code is contributed by chitranayal

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Output:

3

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