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Combinations.java

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package Maths;

/** @see <a href="https://en.wikipedia.org/wiki/Combination">Combination</a> */

public class Combinations {

public static void main(String[] args) {

assert combinations(1, 1) == 1;

assert combinations(10, 5) == 252;

assert combinations(6, 3) == 20;

assert combinations(20, 5) == 15504;

}

/**

* Calculate of factorial

*

* @param n the number

* @return factorial of given number

*/

public static long factorial(int n) {

if (n < 0) {

throw new IllegalArgumentException("number is negative");

}

return n == 0 || n == 1 ? 1 : n * factorial(n - 1);

}

/**

* Calculate combinations

*

* @param n first number

* @param k second number

* @return combinations of given {@code n} and {@code k}

*/

public static long combinations(int n, int k) {

return factorial(n) / (factorial(k) * factorial(n - k));

}

}