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Stein’s Algorithm for finding GCD
• Difficulty Level : Hard
• Last Updated : 31 Mar, 2021

Stein’s algorithm or binary GCD algorithm is an algorithm that computes the greatest common divisor of two non-negative integers. Stein’s algorithm replaces division with arithmetic shifts, comparisons, and subtraction.

Examples:

```Input: a = 17, b = 34
Output : 17

Input: a = 50, b = 49
Output: 1```

Algorithm to find GCD using Stein’s algorithm gcd(a, b)

1. If both a and b are 0, gcd is zero gcd(0, 0) = 0.
2. gcd(a, 0) = a and gcd(0, b) = b because everything divides 0.
3. If a and b are both even, gcd(a, b) = 2*gcd(a/2, b/2) because 2 is a common divisor. Multiplication with 2 can be done with bitwise shift operator.
4. If a is even and b is odd, gcd(a, b) = gcd(a/2, b). Similarly, if a is odd and b is even, then
gcd(a, b) = gcd(a, b/2). It is because 2 is not a common divisor.
5. If both a and b are odd, then gcd(a, b) = gcd(|a-b|/2, b). Note that difference of two odd numbers is even
6. Repeat steps 3–5 until a = b, or until a = 0. In either case, the GCD is power(2, k) * b, where power(2, k) is 2 raise to the power of k and k is the number of common factors of 2 found in step 2.

Iterative Implementation

## C++

 `// Iterative C++ program to``// implement Stein's Algorithm``#include ``using` `namespace` `std;` `// Function to implement``// Stein's Algorithm``int` `gcd(``int` `a, ``int` `b)``{``    ``/* GCD(0, b) == b; GCD(a, 0) == a,``       ``GCD(0, 0) == 0 */``    ``if` `(a == 0)``        ``return` `b;``    ``if` `(b == 0)``        ``return` `a;` `    ``/*Finding K, where K is the``      ``greatest power of 2``      ``that divides both a and b. */``    ``int` `k;``    ``for` `(k = 0; ((a | b) & 1) == 0; ++k)``    ``{``        ``a >>= 1;``        ``b >>= 1;``    ``}` `    ``/* Dividing a by 2 until a becomes odd */``    ``while` `((a & 1) == 0)``        ``a >>= 1;` `    ``/* From here on, 'a' is always odd. */``    ``do``    ``{``        ``/* If b is even, remove all factor of 2 in b */``        ``while` `((b & 1) == 0)``            ``b >>= 1;` `        ``/* Now a and b are both odd.``           ``Swap if necessary so a <= b,``           ``then set b = b - a (which is even).*/``        ``if` `(a > b)``            ``swap(a, b); ``// Swap u and v.` `        ``b = (b - a);``    ``}``while` `(b != 0);` `    ``/* restore common factors of 2 */``    ``return` `a << k;``}` `// Driver code``int` `main()``{``    ``int` `a = 34, b = 17;``    ``printf``(``"Gcd of given numbers is %d\n"``, gcd(a, b));``    ``return` `0;``}`

## Java

 `// Iterative Java program to``// implement Stein's Algorithm``import` `java.io.*;` `class` `GFG {` `    ``// Function to implement Stein's``    ``// Algorithm``    ``static` `int` `gcd(``int` `a, ``int` `b)``    ``{``        ``// GCD(0, b) == b; GCD(a, 0) == a,``        ``// GCD(0, 0) == 0``        ``if` `(a == ``0``)``            ``return` `b;``        ``if` `(b == ``0``)``            ``return` `a;` `        ``// Finding K, where K is the greatest``        ``// power of 2 that divides both a and b``        ``int` `k;``        ``for` `(k = ``0``; ((a | b) & ``1``) == ``0``; ++k)``        ``{``            ``a >>= ``1``;``            ``b >>= ``1``;``        ``}` `        ``// Dividing a by 2 until a becomes odd``        ``while` `((a & ``1``) == ``0``)``            ``a >>= ``1``;` `        ``// From here on, 'a' is always odd.``        ``do``        ``{``            ``// If b is even, remove``            ``// all factor of 2 in b``            ``while` `((b & ``1``) == ``0``)``                ``b >>= ``1``;` `            ``// Now a and b are both odd. Swap``            ``// if necessary so a <= b, then set``            ``// b = b - a (which is even)``            ``if` `(a > b)``            ``{``                ``// Swap u and v.``                ``int` `temp = a;``                ``a = b;``                ``b = temp;``            ``}` `            ``b = (b - a);``        ``} ``while` `(b != ``0``);` `        ``// restore common factors of 2``        ``return` `a << k;``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String args[])``    ``{``        ``int` `a = ``34``, b = ``17``;` `        ``System.out.println(``"Gcd of given "``                           ``+ ``"numbers is "` `+ gcd(a, b));``    ``}``}` `// This code is contributed by Nikita Tiwari`

## Python

 `# Iterative Python 3 program to``# implement Stein's Algorithm` `# Function to implement``# Stein's Algorithm`  `def` `gcd(a, b):` `    ``# GCD(0, b) == b; GCD(a, 0) == a,``    ``# GCD(0, 0) == 0``    ``if` `(a ``=``=` `0``):``        ``return` `b` `    ``if` `(b ``=``=` `0``):``        ``return` `a` `    ``# Finding K, where K is the``    ``# greatest power of 2 that``    ``# divides both a and b.``    ``k ``=` `0` `    ``while``(((a | b) & ``1``) ``=``=` `0``):``        ``a ``=` `a >> ``1``        ``b ``=` `b >> ``1``        ``k ``=` `k ``+` `1` `    ``# Dividing a by 2 until a becomes odd``    ``while` `((a & ``1``) ``=``=` `0``):``        ``a ``=` `a >> ``1` `    ``# From here on, 'a' is always odd.``    ``while``(b !``=` `0``):` `        ``# If b is even, remove all``        ``# factor of 2 in b``        ``while` `((b & ``1``) ``=``=` `0``):``            ``b ``=` `b >> ``1` `        ``# Now a and b are both odd. Swap if``        ``# necessary so a <= b, then set``        ``# b = b - a (which is even).``        ``if` `(a > b):` `            ``# Swap u and v.``            ``temp ``=` `a``            ``a ``=` `b``            ``b ``=` `temp` `        ``b ``=` `(b ``-` `a)` `    ``# restore common factors of 2``    ``return` `(a << k)`  `# Driver code``a ``=` `34``b ``=` `17` `print``(``"Gcd of given numbers is "``, gcd(a, b))` `# This code is contributed by Nikita Tiwari.`

## C#

 `// Iterative C# program to implement``// Stein's Algorithm``using` `System;` `class` `GFG {` `    ``// Function to implement Stein's``    ``// Algorithm``    ``static` `int` `gcd(``int` `a, ``int` `b)``    ``{` `        ``// GCD(0, b) == b; GCD(a, 0) == a,``        ``// GCD(0, 0) == 0``        ``if` `(a == 0)``            ``return` `b;``        ``if` `(b == 0)``            ``return` `a;` `        ``// Finding K, where K is the greatest``        ``// power of 2 that divides both a and b``        ``int` `k;``        ``for` `(k = 0; ((a | b) & 1) == 0; ++k)``        ``{``            ``a >>= 1;``            ``b >>= 1;``        ``}` `        ``// Dividing a by 2 until a becomes odd``        ``while` `((a & 1) == 0)``            ``a >>= 1;` `        ``// From here on, 'a' is always odd``        ``do``        ``{``            ``// If b is even, remove``            ``// all factor of 2 in b``            ``while` `((b & 1) == 0)``                ``b >>= 1;` `            ``/* Now a and b are both odd. Swap``            ``if necessary so a <= b, then set``            ``b = b - a (which is even).*/``            ``if` `(a > b) {` `                ``// Swap u and v.``                ``int` `temp = a;``                ``a = b;``                ``b = temp;``            ``}` `            ``b = (b - a);``        ``} ``while` `(b != 0);` `        ``/* restore common factors of 2 */``        ``return` `a << k;``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main()``    ``{``        ``int` `a = 34, b = 17;` `        ``Console.Write(``"Gcd of given "``                      ``+ ``"numbers is "` `+ gcd(a, b));``    ``}``}` `// This code is contributed by nitin mittal`

## PHP

 `>= 1;``        ``\$b` `>>= 1;``    ``}` `    ``// Dividing a by 2 until a becomes odd``    ``while` `((``\$a` `& 1) == 0)``        ``\$a` `>>= 1;` `    ``// From here on, 'a' is always odd.``    ``do``    ``{``        ` `        ``// If b is even, remove``        ``// all factor of 2 in b``        ``while` `((``\$b` `& 1) == 0)``            ``\$b` `>>= 1;` `        ``// Now a and b are both odd. Swap``        ``// if necessary so a <= b, then set``        ``// b = b - a (which is even)``        ``if` `(``\$a` `> ``\$b``)``            ``swap(``\$a``, ``\$b``); ``// Swap u and v.` `        ``\$b` `= (``\$b` `- ``\$a``);``    ``} ``while` `(``\$b` `!= 0);` `    ``// restore common factors of 2``    ``return` `\$a` `<< ``\$k``;``}` `// Driver code``\$a` `= 34; ``\$b` `= 17;``echo` `"Gcd of given numbers is "` `.``                     ``gcd(``\$a``, ``\$b``);` `// This code is contributed by ajit``?>`

## Javascript

 ``
Output

`Gcd of given numbers is 17`

Recursive Implementation

## C++

 `// Recursive C++ program to``// implement Stein's Algorithm``#include ``using` `namespace` `std;` `// Function to implement``// Stein's Algorithm``int` `gcd(``int` `a, ``int` `b)``{``    ``if` `(a == b)``        ``return` `a;` `    ``// GCD(0, b) == b; GCD(a, 0) == a,``    ``// GCD(0, 0) == 0``    ``if` `(a == 0)``        ``return` `b;``    ``if` `(b == 0)``        ``return` `a;` `    ``// look for factors of 2``    ``if` `(~a & 1) ``// a is even``    ``{``        ``if` `(b & 1) ``// b is odd``            ``return` `gcd(a >> 1, b);``        ``else` `// both a and b are even``            ``return` `gcd(a >> 1, b >> 1) << 1;``    ``}` `    ``if` `(~b & 1) ``// a is odd, b is even``        ``return` `gcd(a, b >> 1);` `    ``// reduce larger number``    ``if` `(a > b)``        ``return` `gcd((a - b) >> 1, b);` `    ``return` `gcd((b - a) >> 1, a);``}` `// Driver code``int` `main()``{``    ``int` `a = 34, b = 17;``    ``printf``(``"Gcd of given numbers is %d\n"``, gcd(a, b));``    ``return` `0;``}`

## Java

 `// Recursive Java program to``// implement Stein's Algorithm``import` `java.io.*;` `class` `GFG {` `    ``// Function to implement``    ``// Stein's Algorithm``    ``static` `int` `gcd(``int` `a, ``int` `b)``    ``{``        ``if` `(a == b)``            ``return` `a;` `        ``// GCD(0, b) == b; GCD(a, 0) == a,``        ``// GCD(0, 0) == 0``        ``if` `(a == ``0``)``            ``return` `b;``        ``if` `(b == ``0``)``            ``return` `a;` `        ``// look for factors of 2``        ``if` `((~a & ``1``) == ``1``) ``// a is even``        ``{``            ``if` `((b & ``1``) == ``1``) ``// b is odd``                ``return` `gcd(a >> ``1``, b);` `            ``else` `// both a and b are even``                ``return` `gcd(a >> ``1``, b >> ``1``) << ``1``;``        ``}` `        ``// a is odd, b is even``        ``if` `((~b & ``1``) == ``1``)``            ``return` `gcd(a, b >> ``1``);` `        ``// reduce larger number``        ``if` `(a > b)``            ``return` `gcd((a - b) >> ``1``, b);` `        ``return` `gcd((b - a) >> ``1``, a);``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String args[])``    ``{``        ``int` `a = ``34``, b = ``17``;``        ``System.out.println(``"Gcd of given"``                           ``+ ``"numbers is "` `+ gcd(a, b));``    ``}``}` `// This code is contributed by Nikita Tiwari`

## Python

 `# Recursive Python 3 program to``# implement Stein's Algorithm` `# Function to implement``# Stein's Algorithm`  `def` `gcd(a, b):` `    ``if` `(a ``=``=` `b):``        ``return` `a` `    ``# GCD(0, b) == b; GCD(a, 0) == a,``    ``# GCD(0, 0) == 0``    ``if` `(a ``=``=` `0``):``        ``return` `b` `    ``if` `(b ``=``=` `0``):``        ``return` `a` `    ``# look for factors of 2``    ``# a is even``    ``if` `((~a & ``1``) ``=``=` `1``):` `        ``# b is odd``        ``if` `((b & ``1``) ``=``=` `1``):``            ``return` `gcd(a >> ``1``, b)``        ``else``:``            ``# both a and b are even``            ``return` `(gcd(a >> ``1``, b >> ``1``) << ``1``)` `    ``# a is odd, b is even``    ``if` `((~b & ``1``) ``=``=` `1``):``        ``return` `gcd(a, b >> ``1``)` `    ``# reduce larger number``    ``if` `(a > b):``        ``return` `gcd((a ``-` `b) >> ``1``, b)` `    ``return` `gcd((b ``-` `a) >> ``1``, a)`  `# Driver code``a, b ``=` `34``, ``17``print``(``"Gcd of given numbers is "``,``      ``gcd(a, b))` `# This code is contributed``# by Nikita Tiwari.`

## C#

 `// Recursive C# program to``// implement Stein's Algorithm``using` `System;` `class` `GFG {` `    ``// Function to implement``    ``// Stein's Algorithm``    ``static` `int` `gcd(``int` `a, ``int` `b)``    ``{``        ``if` `(a == b)``            ``return` `a;` `        ``// GCD(0, b) == b;``        ``// GCD(a, 0) == a,``        ``// GCD(0, 0) == 0``        ``if` `(a == 0)``            ``return` `b;``        ``if` `(b == 0)``            ``return` `a;` `        ``// look for factors of 2``        ``// a is even``        ``if` `((~a & 1) == 1) {` `            ``// b is odd``            ``if` `((b & 1) == 1)``                ``return` `gcd(a >> 1, b);` `            ``else` `                ``// both a and b are even``                ``return` `gcd(a >> 1, b >> 1) << 1;``        ``}` `        ``// a is odd, b is even``        ``if` `((~b & 1) == 1)``            ``return` `gcd(a, b >> 1);` `        ``// reduce larger number``        ``if` `(a > b)``            ``return` `gcd((a - b) >> 1, b);` `        ``return` `gcd((b - a) >> 1, a);``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main()``    ``{``        ``int` `a = 34, b = 17;``        ``Console.Write(``"Gcd of given"``                      ``+ ``"numbers is "` `+ gcd(a, b));``    ``}``}` `// This code is contributed by nitin mittal.`

## PHP

 `> 1, ``\$b``);``        ``else` `// both a and b are even``            ``return` `gcd(``\$a` `>> 1, ``\$b` `>> 1) << 1;``    ``}` `    ``if` `(~``\$b` `& 1) ``// a is odd, b is even``        ``return` `gcd(``\$a``, ``\$b` `>> 1);` `    ``// reduce larger number``    ``if` `(``\$a` `> ``\$b``)``        ``return` `gcd((``\$a` `- ``\$b``) >> 1, ``\$b``);` `    ``return` `gcd((``\$b` `- ``\$a``) >> 1, ``\$a``);``}` `// Driver code``\$a` `= 34; ``\$b` `= 17;``echo` `"Gcd of given numbers is: "``,``                     ``gcd(``\$a``, ``\$b``);` `// This code is contributed by aj_36``?>`

## Javascript

 ``
Output
`Gcd of given numbers is 17`

Time Complexity: O(N*N) where N is the number of bits in the larger number.
You may also like – Basic and Extended Euclidian Algorithm
References

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