extras: add P16-P23, P25

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

P18.java

@@ -0,0 +1,11 @@
+public class P18 {
+  public static void main(String[] _args) {
+      int a = 5, b, f;
+      b = f = a;
+
+      while (b-- > 1)
+          f *= b;
+
+      System.out.println(a + "! = " + f);
+  }
+}