* factorial using using iteration

* add ceil() to maths
* add combinations() to maths
* add floor() to maths

Maths/Combinations.java

@@ -0,0 +1,37 @@
+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));
+    }
+}