Implement challenge 19 in Rust so that I can move on with good conscience.

src/main.rs

@@ -9,7 +9,7 @@ mod set2;
 mod set3;
 
 fn main() {
-    const RUN_ALL: bool = false;
+    const RUN_ALL: bool = true;
     if RUN_ALL {
         set1::challenge1();
         set1::challenge2();
@@ -32,6 +32,6 @@ fn main() {
         set3::challenge19();
         set3::challenge20();
     } else {
-        set3::challenge19();
+        set3::challenge20();
     }
 }