extras: add P16-P23, P25

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

P20.java

@@ -0,0 +1,14 @@
+public class P20 {
+    public static void main(String[] args) {
+        int n = 10;
+        System.out.print("Fibonacci Series of length " + n + ": ");
+        fibonacci(0, 1, n);
+    }
+
+    public static void fibonacci(int a, int b, int n) {
+        if (n == 0)
+            return;
+        System.out.print(a + " ");
+        fibonacci(b, a + b, n - 1);
+    }
+}