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# Common Divisors of Two Numbers

• Difficulty Level : Easy
• Last Updated : 29 Jun, 2021

Given two integer numbers, the task is to find count of all common divisors of given numbers?

Examples :

```Input : a = 12, b = 24
Output: 6
// all common divisors are 1, 2, 3,
// 4, 6 and 12

Input : a = 3, b = 17
Output: 1
// all common divisors are 1

Input : a = 20, b = 36
Output: 3
// all common divisors are 1, 2, 4```

It is recommended to refer all divisors of a given number as a prerequisite of this article.

Naive Solution
A simple solution is to first find all divisors of first number and store them in an array or hash. Then find common divisors of second number and store them. Finally print common elements of two stored arrays or hash. The key is that the magnitude of powers of prime factors of a divisor should be equal to the minimum power of two prime factors of a and b.

• Find the prime factors of a using prime factorization.
• Find the count of each prime factor of a and store it in a Hashmap.
• Prime factorize b using distinct prime factors of a.
• Then the total number of divisors would be equal to the product of (count + 1)
of each factor.
• count is the minimum of counts of each prime factors of a and b.
• This gives the count of all divisors of a and b.

## C++

 `// C++ implementation of program``#include ``using` `namespace` `std;` `// Map to store the count of each``// prime factor of a``map<``int``, ``int``> ma;` `// Function that calculate the count of``// each prime factor of a number``void` `primeFactorize(``int` `a)``{``    ``for``(``int` `i = 2; i * i <= a; i += 2)``    ``{``        ``int` `cnt = 0;``        ``while` `(a % i == 0)``        ``{``            ``cnt++;``            ``a /= i;``        ``}``        ``ma[i] = cnt;``    ``}``    ``if` `(a > 1)``    ``{``        ``ma[a] = 1;``    ``}``}` `// Function to calculate all common``// divisors of two given numbers``// a, b --> input integer numbers``int` `commDiv(``int` `a, ``int` `b)``{``    ` `    ``// Find count of each prime factor of a``    ``primeFactorize(a);` `    ``// stores number of common divisors``    ``int` `res = 1;` `    ``// Find the count of prime factors``    ``// of b using distinct prime factors of a``    ``for``(``auto` `m = ma.begin();``             ``m != ma.end(); m++)``    ``{``        ``int` `cnt = 0;``        ``int` `key = m->first;``        ``int` `value = m->second;` `        ``while` `(b % key == 0)``        ``{``            ``b /= key;``            ``cnt++;``        ``}` `        ``// Prime factor of common divisor``        ``// has minimum cnt of both a and b``        ``res *= (min(cnt, value) + 1);``    ``}``    ``return` `res;``}` `// Driver code   ``int` `main()``{``    ``int` `a = 12, b = 24;``    ` `    ``cout << commDiv(a, b) << endl;``    ` `    ``return` `0;``}` `// This code is contributed by divyeshrabadiya07`

## Java

 `// Java implementation of program``import` `java.util.*;``import` `java.io.*;` `class` `GFG {``    ``// map to store the count of each prime factor of a``    ``static` `HashMap ma = ``new` `HashMap<>();` `    ``// method that calculate the count of``    ``// each prime factor of a number``    ``static` `void` `primeFactorize(``int` `a)``    ``{``        ``for` `(``int` `i = ``2``; i * i <= a; i += ``2``) {``            ``int` `cnt = ``0``;``            ``while` `(a % i == ``0``) {``                ``cnt++;``                ``a /= i;``            ``}``            ``ma.put(i, cnt);``        ``}``        ``if` `(a > ``1``)``            ``ma.put(a, ``1``);``    ``}` `    ``// method to calculate all common divisors``    ``// of two given numbers``    ``// a, b --> input integer numbers``    ``static` `int` `commDiv(``int` `a, ``int` `b)``    ``{``        ``// Find count of each prime factor of a``        ``primeFactorize(a);` `        ``// stores number of common divisors``        ``int` `res = ``1``;` `        ``// Find the count of prime factors of b using``        ``// distinct prime factors of a``        ``for` `(Map.Entry m : ma.entrySet()) {``            ``int` `cnt = ``0``;` `            ``int` `key = m.getKey();``            ``int` `value = m.getValue();` `            ``while` `(b % key == ``0``) {``                ``b /= key;``                ``cnt++;``            ``}` `            ``// prime factor of common divisor``            ``// has minimum cnt of both a and b``            ``res *= (Math.min(cnt, value) + ``1``);``        ``}``        ``return` `res;``    ``}` `    ``// Driver method``    ``public` `static` `void` `main(String args[])``    ``{``        ``int` `a = ``12``, b = ``24``;``        ``System.out.println(commDiv(a, b));``    ``}``}`

## Python3

 `# Python3 implementation of program``import` `math` `# Map to store the count of each``# prime factor of a``ma ``=` `{}` `# Function that calculate the count of``# each prime factor of a number``def` `primeFactorize(a):``    ` `    ``sqt ``=` `int``(math.sqrt(a))``    ``for` `i ``in` `range``(``2``, sqt, ``2``):``        ``cnt ``=` `0``        ` `        ``while` `(a ``%` `i ``=``=` `0``):``            ``cnt ``+``=` `1``            ``a ``/``=` `i``            ` `        ``ma[i] ``=` `cnt``        ` `    ``if` `(a > ``1``):``        ``ma[a] ``=` `1``        ` `# Function to calculate all common``# divisors of two given numbers``# a, b --> input integer numbers``def` `commDiv(a, b):``    ` `    ``# Find count of each prime factor of a``    ``primeFactorize(a)``    ` `    ``# stores number of common divisors``    ``res ``=` `1``    ` `    ``# Find the count of prime factors``    ``# of b using distinct prime factors of a``    ``for` `key, value ``in` `ma.items():``        ``cnt ``=` `0``        ` `        ``while` `(b ``%` `key ``=``=` `0``):``            ``b ``/``=` `key``            ``cnt ``+``=` `1``            ` `        ``# Prime factor of common divisor``        ``# has minimum cnt of both a and b``        ``res ``*``=` `(``min``(cnt, value) ``+` `1``)``        ` `    ``return` `res``    ` `# Driver code   ``a ``=` `12``b ``=` `24` `print``(commDiv(a, b))` `# This code is contributed by Stream_Cipher`

## C#

 `// C# implementation of program``using` `System;``using` `System.Collections.Generic;` `class` `GFG{``  ` `// Map to store the count of each``// prime factor of a``static` `Dictionary<``int``,``                  ``int``> ma = ``new` `Dictionary<``int``,``                                           ``int``>();` `// Function that calculate the count of``// each prime factor of a number``static` `void` `primeFactorize(``int` `a)``{``    ``for``(``int` `i = 2; i * i <= a; i += 2)``    ``{``        ``int` `cnt = 0;``        ``while` `(a % i == 0)``        ``{``            ``cnt++;``            ``a /= i;``        ``}``        ``ma.Add(i, cnt);``    ``}``    ` `    ``if` `(a > 1)``        ``ma.Add(a, 1);``}` `// Function to calculate all common``// divisors of two given numbers``// a, b --> input integer numbers``static` `int` `commDiv(``int` `a, ``int` `b)``{``    ` `    ``// Find count of each prime factor of a``    ``primeFactorize(a);``    ` `    ``// Stores number of common divisors``    ``int` `res = 1;``    ` `    ``// Find the count of prime factors``    ``// of b using distinct prime factors of a``    ``foreach``(KeyValuePair<``int``, ``int``> m ``in` `ma)``    ``{``        ``int` `cnt = 0;``        ``int` `key = m.Key;``        ``int` `value = m.Value;``        ` `        ``while` `(b % key == 0)``        ``{``            ``b /= key;``            ``cnt++;``        ``}` `        ``// Prime factor of common divisor``        ``// has minimum cnt of both a and b``        ``res *= (Math.Min(cnt, value) + 1);``    ``}``    ``return` `res;``}` `// Driver code   ``static` `void` `Main()``{``    ``int` `a = 12, b = 24;``    ` `    ``Console.WriteLine(commDiv(a, b));``}``}` `// This code is contributed by divyesh072019`

## Javascript

 ``

Output:

`6`

Efficient Solution –
A better solution is to calculate the greatest common divisor (gcd) of given two numbers, and then count divisors of that gcd.

## C++

 `// C++ implementation of program``#include ``using` `namespace` `std;` `// Function to calculate gcd of two numbers``int` `gcd(``int` `a, ``int` `b)``{``    ``if` `(a == 0)``        ``return` `b;``    ``return` `gcd(b % a, a);``}` `// Function to calculate all common divisors``// of two given numbers``// a, b --> input integer numbers``int` `commDiv(``int` `a, ``int` `b)``{``    ``// find gcd of a, b``    ``int` `n = gcd(a, b);` `    ``// Count divisors of n.``    ``int` `result = 0;``    ``for` `(``int` `i = 1; i <= ``sqrt``(n); i++) {``        ``// if 'i' is factor of n``        ``if` `(n % i == 0) {``            ``// check if divisors are equal``            ``if` `(n / i == i)``                ``result += 1;``            ``else``                ``result += 2;``        ``}``    ``}``    ``return` `result;``}` `// Driver program to run the case``int` `main()``{``    ``int` `a = 12, b = 24;``    ``cout << commDiv(a, b);``    ``return` `0;``}`

## Java

 `// Java implementation of program` `class` `Test {``    ``// method to calculate gcd of two numbers``    ``static` `int` `gcd(``int` `a, ``int` `b)``    ``{``        ``if` `(a == ``0``)``            ``return` `b;` `        ``return` `gcd(b % a, a);``    ``}``    ``// method to calculate all common divisors``    ``// of two given numbers``    ``// a, b --> input integer numbers``    ``static` `int` `commDiv(``int` `a, ``int` `b)``    ``{``        ``// find gcd of a, b``        ``int` `n = gcd(a, b);` `        ``// Count divisors of n.``        ``int` `result = ``0``;``        ``for` `(``int` `i = ``1``; i <= Math.sqrt(n); i++) {``            ``// if 'i' is factor of n``            ``if` `(n % i == ``0``) {``                ``// check if divisors are equal``                ``if` `(n / i == i)``                    ``result += ``1``;``                ``else``                    ``result += ``2``;``            ``}``        ``}``        ``return` `result;``    ``}` `    ``// Driver method``    ``public` `static` `void` `main(String args[])``    ``{``        ``int` `a = ``12``, b = ``24``;``        ``System.out.println(commDiv(a, b));``    ``}``}`

## Python3

 `# Python implementation of program``from` `math ``import` `sqrt`  `# Function to calculate gcd of two numbers``def` `gcd(a, b):``    ` `    ``if` `a ``=``=` `0``:``        ``return` `b``    ``return` `gcd(b ``%` `a, a)``  ` `# Function to calculate all common divisors``# of two given numbers``# a, b --> input integer numbers``def` `commDiv(a, b):``    ` `    ``# find GCD of a, b``    ``n ``=` `gcd(a, b)` `    ``# Count divisors of n``    ``result ``=` `0``    ``for` `i ``in` `range``(``1``,``int``(sqrt(n))``+``1``):` `        ``# if i is a factor of n``        ``if` `n ``%` `i ``=``=` `0``:` `            ``# check if divisors are equal``            ``if` `n``/``i ``=``=` `i:``                ``result ``+``=` `1``            ``else``:``                ``result ``+``=` `2``                ` `    ``return` `result` `# Driver program to run the case``if` `__name__ ``=``=` `"__main__"``:``    ``a ``=` `12``    ``b ``=` `24``;``    ``print``(commDiv(a, b))`

## C#

 `// C# implementation of program``using` `System;` `class` `GFG {` `    ``// method to calculate gcd``    ``// of two numbers``    ``static` `int` `gcd(``int` `a, ``int` `b)``    ``{``        ``if` `(a == 0)``            ``return` `b;` `        ``return` `gcd(b % a, a);``    ``}` `    ``// method to calculate all``    ``// common divisors of two``    ``// given numbers a, b -->``    ``// input integer numbers``    ``static` `int` `commDiv(``int` `a, ``int` `b)``    ``{` `        ``// find gcd of a, b``        ``int` `n = gcd(a, b);` `        ``// Count divisors of n.``        ``int` `result = 0;``        ``for` `(``int` `i = 1; i <= Math.Sqrt(n); i++) {` `            ``// if 'i' is factor of n``            ``if` `(n % i == 0) {` `                ``// check if divisors are equal``                ``if` `(n / i == i)``                    ``result += 1;``                ``else``                    ``result += 2;``            ``}``        ``}` `        ``return` `result;``    ``}` `    ``// Driver method``    ``public` `static` `void` `Main(String[] args)``    ``{` `        ``int` `a = 12, b = 24;` `        ``Console.Write(commDiv(a, b));``    ``}``}` `// This code contributed by parashar.`

## PHP

 ` input integer numbers``function` `commDiv(``\$a``, ``\$b``)``{``    ``// find gcd of a, b``    ``\$n` `= gcd(``\$a``, ``\$b``);` `    ``// Count divisors of n.``    ``\$result` `= 0;``    ``for` `(``\$i` `= 1; ``\$i` `<= sqrt(``\$n``);``                 ``\$i``++)``    ``{``        ``// if 'i' is factor of n``        ``if` `(``\$n` `% ``\$i` `== 0)``        ``{``            ``// check if divisors``            ``// are equal``            ``if` `(``\$n` `/ ``\$i` `== ``\$i``)``                ``\$result` `+= 1;``            ``else``                ``\$result` `+= 2;``        ``}``    ``}``    ``return` `\$result``;``}` `// Driver Code``\$a` `= 12; ``\$b` `= 24;``echo``(commDiv(``\$a``, ``\$b``));` `// This code is contributed by Ajit.``?>`

## Javascript

 ``

Output :

`6`

Time complexity: O(log(n)+ n1/2) where n is the gcd of two numbers.

This article is contributed by Shashank Mishra ( Gullu ). If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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