# Euclidean algorithms (Basic and Extended)

The Euclidean algorithm is a way to find the greatest common divisor of two positive integers. GCD of two numbers is the largest number that divides both of them. A simple way to find GCD is to factorize both numbers and multiply common prime factors.

**Basic Euclidean Algorithm for GCD: **

The algorithm is based on the below facts.

- If we subtract a smaller number from a larger one (we reduce a larger number), GCD doesn’t change. So if we keep subtracting repeatedly the larger of two, we end up with GCD.
- Now instead of subtraction, if we divide the smaller number, the algorithm stops when we find the remainder 0.

Below is a recursive function to evaluate gcd using Euclid’s algorithm:

## C

`// C program to demonstrate Basic Euclidean Algorithm` `#include <stdio.h>` `// Function to return gcd of a and b` `int` `gcd(` `int` `a, ` `int` `b)` `{` ` ` `if` `(a == 0)` ` ` `return` `b;` ` ` `return` `gcd(b % a, a);` `}` `// Driver code` `int` `main()` `{` ` ` `int` `a = 10, b = 15;` ` ` ` ` `// Function call` ` ` `printf` `(` `"GCD(%d, %d) = %d\n"` `, a, b, gcd(a, b));` ` ` `a = 35, b = 10;` ` ` `printf` `(` `"GCD(%d, %d) = %d\n"` `, a, b, gcd(a, b));` ` ` `a = 31, b = 2;` ` ` `printf` `(` `"GCD(%d, %d) = %d\n"` `, a, b, gcd(a, b));` ` ` `return` `0;` `}` |

## CPP

`// C++ program to demonstrate` `// Basic Euclidean Algorithm` `#include <bits/stdc++.h>` `using` `namespace` `std;` `// Function to return` `// gcd of a and b` `int` `gcd(` `int` `a, ` `int` `b)` `{` ` ` `if` `(a == 0)` ` ` `return` `b;` ` ` `return` `gcd(b % a, a);` `}` `// Driver Code` `int` `main()` `{` ` ` `int` `a = 10, b = 15;` ` ` ` ` `// Function call` ` ` `cout << ` `"GCD("` `<< a << ` `", "` `<< b << ` `") = "` `<< gcd(a, b)` ` ` `<< endl;` ` ` `a = 35, b = 10;` ` ` `cout << ` `"GCD("` `<< a << ` `", "` `<< b << ` `") = "` `<< gcd(a, b)` ` ` `<< endl;` ` ` `a = 31, b = 2;` ` ` `cout << ` `"GCD("` `<< a << ` `", "` `<< b << ` `") = "` `<< gcd(a, b)` ` ` `<< endl;` ` ` `return` `0;` `}` |

## Java

`// Java program to demonstrate Basic Euclidean Algorithm` `import` `java.lang.*;` `import` `java.util.*;` `class` `GFG {` ` ` `// extended Euclidean Algorithm` ` ` `public` `static` `int` `gcd(` `int` `a, ` `int` `b)` ` ` `{` ` ` `if` `(a == ` `0` `)` ` ` `return` `b;` ` ` `return` `gcd(b % a, a);` ` ` `}` ` ` `// Driver code` ` ` `public` `static` `void` `main(String[] args)` ` ` `{` ` ` `int` `a = ` `10` `, b = ` `15` `, g;` ` ` ` ` `// Function call` ` ` `g = gcd(a, b);` ` ` `System.out.println(` `"GCD("` `+ a + ` `" , "` `+ b` ` ` `+ ` `") = "` `+ g);` ` ` `a = ` `35` `;` ` ` `b = ` `10` `;` ` ` `g = gcd(a, b);` ` ` `System.out.println(` `"GCD("` `+ a + ` `" , "` `+ b` ` ` `+ ` `") = "` `+ g);` ` ` `a = ` `31` `;` ` ` `b = ` `2` `;` ` ` `g = gcd(a, b);` ` ` `System.out.println(` `"GCD("` `+ a + ` `" , "` `+ b` ` ` `+ ` `") = "` `+ g);` ` ` `}` `}` `// Code Contributed by Mohit Gupta_OMG <(0_o)>` |

## Python3

`# Python3 program to demonstrate Basic Euclidean Algorithm` `# Function to return gcd of a and b` `def` `gcd(a, b):` ` ` `if` `a ` `=` `=` `0` `:` ` ` `return` `b` ` ` `return` `gcd(b ` `%` `a, a)` `# Driver code` `if` `__name__ ` `=` `=` `"__main__"` `:` ` ` `a ` `=` `10` ` ` `b ` `=` `15` ` ` `print` `(` `"gcd("` `, a, ` `","` `, b, ` `") = "` `, gcd(a, b))` ` ` `a ` `=` `35` ` ` `b ` `=` `10` ` ` `print` `(` `"gcd("` `, a, ` `","` `, b, ` `") = "` `, gcd(a, b))` ` ` `a ` `=` `31` ` ` `b ` `=` `2` ` ` `print` `(` `"gcd("` `, a, ` `","` `, b, ` `") = "` `, gcd(a, b))` `# Code Contributed By Mohit Gupta_OMG <(0_o)>` |

## C#

`// C# program to demonstrate Basic Euclidean Algorithm` `using` `System;` `class` `GFG {` ` ` `public` `static` `int` `gcd(` `int` `a, ` `int` `b)` ` ` `{` ` ` `if` `(a == 0)` ` ` `return` `b;` ` ` `return` `gcd(b % a, a);` ` ` `}` ` ` `// Driver Code` ` ` `static` `public` `void` `Main()` ` ` `{` ` ` `int` `a = 10, b = 15, g;` ` ` `g = gcd(a, b);` ` ` `Console.WriteLine(` `"GCD("` `+ a + ` `" , "` `+ b` ` ` `+ ` `") = "` `+ g);` ` ` `a = 35;` ` ` `b = 10;` ` ` `g = gcd(a, b);` ` ` `Console.WriteLine(` `"GCD("` `+ a + ` `" , "` `+ b` ` ` `+ ` `") = "` `+ g);` ` ` `a = 31;` ` ` `b = 2;` ` ` `g = gcd(a, b);` ` ` `Console.WriteLine(` `"GCD("` `+ a + ` `" , "` `+ b` ` ` `+ ` `") = "` `+ g);` ` ` `}` `}` `// This code is contributed by ajit` |

## PHP

`// php program to demonstrate Basic Euclidean Algorithm` `<?php` `// PHP program to demonstrate` `// Basic Euclidean Algorithm` `// Function to return` `// gcd of a and b` `function` `gcd(` `$a` `, ` `$b` `)` `{` ` ` `if` `(` `$a` `== 0)` ` ` `return` `$b` `;` ` ` `return` `gcd(` `$b` `% ` `$a` `, ` `$a` `);` `}` `// Driver Code` `$a` `= 10; ` `$b` `= 15;` `// Function call` `echo` `"GCD("` `,` `$a` `,` `","` `, ` `$b` `,` `") = "` `, ` ` ` `gcd(` `$a` `, ` `$b` `);` `echo` `"\n"` `;` `$a` `= 35; ` `$b` `= 10;` `echo` `"GCD("` `,` `$a` `,` `","` `,` `$b` `,` `") = "` `, ` ` ` `gcd(` `$a` `, ` `$b` `);` `echo` `"\n"` `;` `$a` `= 31; ` `$b` `= 2;` `echo` `"GCD("` `,` `$a` `,` `","` `, ` `$b` `,` `") = "` `, ` ` ` `gcd(` `$a` `, ` `$b` `);` `// This code is contributed by m_kit` `?>` |

## Javascript

`// JavaScript program to demonstrate` `// Basic Euclidean Algorithm` `// Function to return ` `// gcd of a and b` `function` `gcd( a, b)` `{` ` ` `if` `(a == 0)` ` ` `return` `b;` ` ` `return` `gcd(b % a, a);` `}` `// Driver Code` ` ` `let a = 10, b = 15;` ` ` `document.write( ` `"GCD("` `+ a + ` `", "` ` ` `+ b + ` `") = "` `+ gcd(a, b) +` `"<br/>"` `);` ` ` ` ` `a = 35, b = 10;` ` ` `document.write( ` `"GCD("` `+ a + ` `", "` ` ` `+ b + ` `") = "` `+ gcd(a, b) +` `"<br/>"` `);` ` ` ` ` `a = 31, b = 2;` ` ` `document.write( ` `"GCD("` `+ a + ` `", "` ` ` `+ b + ` `") = "` `+ gcd(a, b) +` `"<br/>"` `);` `// This code contributed by aashish1995 ` |

**Output**

GCD(10, 15) = 5 GCD(35, 10) = 5 GCD(31, 2) = 1

**Time Complexity:** O(Log min(a, b))**Auxiliary Space: **O(Log (min(a,b))

**Extended Euclidean Algorithm: **

Extended Euclidean algorithm also finds integer coefficients x and y such that: ax + by = gcd(a, b)

**Examples:**

Input:a = 30, b = 20Output:gcd = 10, x = 1, y = -1

(Note that 30*1 + 20*(-1) = 10)

Input:a = 35, b = 15Output:gcd = 5, x = 1, y = -2

(Note that 35*1 + 15*(-2) = 5)

The extended Euclidean algorithm updates the results of gcd(a, b) using the results calculated by the recursive call gcd(b%a, a). Let values of x and y calculated by the recursive call be x_{1} and y_{1}. x and y are updated using the below expressions.

ax + by = gcd(a, b)

gcd(a, b) = gcd(b%a, a)

gcd(b%a, a) = (b%a)x_{1 }+ ay_{1}

ax + by = (b%a)x_{1 }+ ay_{1}

ax + by = (b – [b/a] * a)x_{1 }+ ay_{1}

ax + by = a(y_{1}– [b/a] * x_{1}) + bx_{1}Comparing LHS and RHS,

x = y_{1}– ⌊b/a⌋ * x_{1}

y = x_{1}

Below is an implementation of the above approach:

## C++

`// C++ program to demonstrate working of ` `// extended Euclidean Algorithm ` `#include <bits/stdc++.h> ` `using` `namespace` `std;` `// Function for extended Euclidean Algorithm ` `int` `gcdExtended(` `int` `a, ` `int` `b, ` `int` `*x, ` `int` `*y) ` `{ ` ` ` `// Base Case ` ` ` `if` `(a == 0) ` ` ` `{ ` ` ` `*x = 0; ` ` ` `*y = 1; ` ` ` `return` `b; ` ` ` `} ` ` ` `int` `x1, y1; ` `// To store results of recursive call ` ` ` `int` `gcd = gcdExtended(b%a, a, &x1, &y1); ` ` ` `// Update x and y using results of ` ` ` `// recursive call ` ` ` `*x = y1 - (b/a) * x1; ` ` ` `*y = x1; ` ` ` `return` `gcd; ` `} ` `// Driver Code` `int` `main() ` `{ ` ` ` `int` `x, y, a = 35, b = 15; ` ` ` `int` `g = gcdExtended(a, b, &x, &y); ` ` ` `cout << ` `"GCD("` `<< a << ` `", "` `<< b ` ` ` `<< ` `") = "` `<< g << endl;` ` ` `return` `0; ` `} ` |

## C

`// C program to demonstrate working of extended` `// Euclidean Algorithm` `#include <stdio.h>` `// C function for extended Euclidean Algorithm` `int` `gcdExtended(` `int` `a, ` `int` `b, ` `int` `*x, ` `int` `*y)` `{` ` ` `// Base Case` ` ` `if` `(a == 0)` ` ` `{` ` ` `*x = 0;` ` ` `*y = 1;` ` ` `return` `b;` ` ` `}` ` ` `int` `x1, y1; ` `// To store results of recursive call` ` ` `int` `gcd = gcdExtended(b%a, a, &x1, &y1);` ` ` `// Update x and y using results of recursive` ` ` `// call` ` ` `*x = y1 - (b/a) * x1;` ` ` `*y = x1;` ` ` `return` `gcd;` `}` `// Driver Program` `int` `main()` `{` ` ` `int` `x, y;` ` ` `int` `a = 35, b = 15;` ` ` `int` `g = gcdExtended(a, b, &x, &y);` ` ` `printf` `(` `"gcd(%d, %d) = %d"` `, a, b, g);` ` ` `return` `0;` `}` |

## Java

`// Java program to demonstrate working of extended` `// Euclidean Algorithm` `import` `java.lang.*;` `import` `java.util.*;` `class` `GFG {` ` ` `// extended Euclidean Algorithm` ` ` `public` `static` `int` `gcdExtended(` `int` `a, ` `int` `b, ` `int` `x,` ` ` `int` `y)` ` ` `{` ` ` `// Base Case` ` ` `if` `(a == ` `0` `) {` ` ` `x = ` `0` `;` ` ` `y = ` `1` `;` ` ` `return` `b;` ` ` `}` ` ` `int` `x1 = ` `1` `,` ` ` `y1 = ` `1` `; ` `// To store results of recursive call` ` ` `int` `gcd = gcdExtended(b % a, a, x1, y1);` ` ` `// Update x and y using results of recursive` ` ` `// call` ` ` `x = y1 - (b / a) * x1;` ` ` `y = x1;` ` ` `return` `gcd;` ` ` `}` ` ` `// Driver Program` ` ` `public` `static` `void` `main(String[] args)` ` ` `{` ` ` `int` `x = ` `1` `, y = ` `1` `;` ` ` `int` `a = ` `35` `, b = ` `15` `;` ` ` `int` `g = gcdExtended(a, b, x, y);` ` ` `System.out.print(` `"gcd("` `+ a + ` `" , "` `+ b` ` ` `+ ` `") = "` `+ g);` ` ` `}` `}` |

## Python3

`# Python program to demonstrate working of extended` `# Euclidean Algorithm` `# function for extended Euclidean Algorithm` `def` `gcdExtended(a, b):` ` ` `# Base Case` ` ` `if` `a ` `=` `=` `0` `:` ` ` `return` `b, ` `0` `, ` `1` ` ` `gcd, x1, y1 ` `=` `gcdExtended(b ` `%` `a, a)` ` ` `# Update x and y using results of recursive` ` ` `# call` ` ` `x ` `=` `y1 ` `-` `(b` `/` `/` `a) ` `*` `x1` ` ` `y ` `=` `x1` ` ` `return` `gcd, x, y` `# Driver code` `a, b ` `=` `35` `, ` `15` `g, x, y ` `=` `gcdExtended(a, b)` `print` `(` `"gcd("` `, a, ` `","` `, b, ` `") = "` `, g)` |

## C#

`// C# program to demonstrate working ` `// of extended Euclidean Algorithm` `using` `System;` `class` `GFG` `{` ` ` ` ` `// extended Euclidean Algorithm` ` ` `public` `static` `int` `gcdExtended(` `int` `a, ` `int` `b, ` ` ` `int` `x, ` `int` `y)` ` ` `{` ` ` `// Base Case` ` ` `if` `(a == 0)` ` ` `{` ` ` `x = 0;` ` ` `y = 1;` ` ` `return` `b;` ` ` `}` ` ` `// To store results of` ` ` `// recursive call` ` ` `int` `x1 = 1, y1 = 1; ` ` ` `int` `gcd = gcdExtended(b % a, a, x1, y1);` ` ` `// Update x and y using ` ` ` `// results of recursive call` ` ` `x = y1 - (b / a) * x1;` ` ` `y = x1;` ` ` `return` `gcd;` ` ` `}` ` ` ` ` `// Driver Code` ` ` `static` `public` `void` `Main ()` ` ` `{` ` ` `int` `x = 1, y = 1;` ` ` `int` `a = 35, b = 15;` ` ` `int` `g = gcdExtended(a, b, x, y);` ` ` `Console.WriteLine(` `"gcd("` `+ a + ` `" , "` `+ ` ` ` `b + ` `") = "` `+ g);` ` ` `}` `}` |

## PHP

`<?php` `// PHP program to demonstrate ` `// working of extended ` `// Euclidean Algorithm` `// PHP function for ` `// extended Euclidean ` `// Algorithm` `function` `gcdExtended(` `$a` `, ` `$b` `, ` ` ` `$x` `, ` `$y` `)` `{` ` ` `// Base Case` ` ` `if` `(` `$a` `== 0)` ` ` `{` ` ` `$x` `= 0;` ` ` `$y` `= 1;` ` ` `return` `$b` `;` ` ` `}` ` ` `// To store results ` ` ` `// of recursive call` ` ` `$gcd` `= gcdExtended(` `$b` `% ` `$a` `, ` ` ` `$a` `, ` `$x` `, ` `$y` `);` ` ` `// Update x and y using` ` ` `// results of recursive` ` ` `// call` ` ` `$x` `= ` `$y` `- ` `floor` `(` `$b` `/ ` `$a` `) * ` `$x` `;` ` ` `$y` `= ` `$x` `;` ` ` `return` `$gcd` `;` `}` `// Driver Code` `$x` `= 0;` `$y` `= 0;` `$a` `= 35; ` `$b` `= 15;` `$g` `= gcdExtended(` `$a` `, ` `$b` `, ` `$x` `, ` `$y` `);` `echo` `"gcd("` `,` `$a` `;` `echo` `", "` `, ` `$b` `, ` `")"` `;` `echo` `" = "` `, ` `$g` `;` `?>` |

## Javascript

`<script>` `// Javascript program to demonstrate ` `// working of extended ` `// Euclidean Algorithm` `// Javascript function for ` `// extended Euclidean ` `// Algorithm` `function` `gcdExtended(a, b, ` ` ` `x, y)` `{` ` ` `// Base Case` ` ` `if` `(a == 0)` ` ` `{` ` ` `x = 0;` ` ` `y = 1;` ` ` `return` `b;` ` ` `}` ` ` `// To store results ` ` ` `// of recursive call` ` ` `let gcd = gcdExtended(b % a, ` ` ` `a, x, y);` ` ` `// Update x and y using` ` ` `// results of recursive` ` ` `// call` ` ` `x = y - (b / a) * x;` ` ` `y = x;` ` ` `return` `gcd;` `}` `// Driver Code` `let x = 0;` `let y = 0;` `let a = 35; ` `let b = 15;` `let g = gcdExtended(a, b, x, y);` `document.write(` `"gcd("` `+ a);` `document.write(` `", "` `+ b + ` `")"` `);` `document.write(` `" = "` `+ g);` `</script>` |

**Output :**

gcd(35, 15) = 5

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

**How does Extended Algorithm Work?**

As seen above, x and y are results for inputs a and b,

a.x + b.y = gcd —-(1)

And x

_{1}and y_{1}are results for inputs b%a and a(b%a).x

_{1}+ a.y_{1}= gcdWhen we put b%a = (b – (⌊b/a⌋).a) in above,

we get following. Note that ⌊b/a⌋ is floor(b/a)(b – (⌊b/a⌋).a).x

_{1}+ a.y_{1}= gcdAbove equation can also be written as below

b.x

_{1}+ a.(y_{1}– (⌊b/a⌋).x_{1}) = gcd —(2)After comparing coefficients of ‘a’ and ‘b’ in (1) and

(2), we get following,

x = y_{1}– ⌊b/a⌋ * x_{1}

y = x_{1}

**How is Extended Algorithm Useful?**

The extended Euclidean algorithm is particularly useful when a and b are coprime (or gcd is 1). Since x is the modular multiplicative inverse of “a modulo b”, and y is the modular multiplicative inverse of “b modulo a”. In particular, the computation of the modular multiplicative inverse is an essential step in RSA public-key encryption method.

This article is contributed by **Ankur**. Please write comments if you find anything incorrect, or if you want to share more information about the topic discussed above