add factorial

java/Stack/factorial.java

@@ -0,0 +1,19 @@
+class factorial {
+	/** @return n! */
+	static long fact(int n) {
+		// To fit n! in a long variable, require n < 21
+		assert (n >= 0) && (n <= 20) : "n out of range";
+		// Make a stack just big enough
+		Stack<Integer> S = new AStack<Integer>(n);
+		while (n > 1) S.push(n--);
+		long result = 1;
+		while (S.length() > 0)
+			result = result * S.pop();
+
+		return result;
+	}
+
+	public static void main(String[] args) {
+		System.out.println(fact(5));
+	}
+}