Implement set 1 challenge 2.

src/main.rs

@@ -1,7 +1,11 @@
 mod set1;
 
 use crate::set1::challenge1;
+use crate::set1::challenge2;
+use crate::set1::challenge3;
 
 fn main() {
     challenge1();
+    challenge2();
+    challenge3();
 }