Updated with required fixes.

Notes:

• Please add todo!() to the function body in the solution setup so it compiles out of the box.
• Rust assertions are not designed for a scenario in which the author of the tested code (the user) does not see the test code. As such, the user doesn't know what "left" and "right" refer to. Please add a clarification to all assertion messages.
• String and &str can be compared directly, no need to call to_string() on your literals for this purpose.
• Building a random string by repeatedly iterating over chars() is wasteful. Create a vector of the base chars and randomly sample from that.