Merge pull request #209 from zongzi531/master

feat: add variables solutions

solutions/variables.md

@@ -42,6 +42,18 @@ fn define_x() -> String {
 }
 ```
 
+```rust
+fn main() {
+    let x = define_x();
+    println!("{:?}, world", x); 
+}
+
+fn define_x() -> &'static str {
+    let x = "hello";
+    x
+}
+```
+
 5.
 ```rust
 fn main() {