Exercise: Fibonacci

The Fibonacci sequence begins with [0,1]. For n>1, the n’th Fibonacci number is calculated recursively as the sum of the n-1’th and n-2’th Fibonacci numbers.

Write a function fib(n) that calculates the n’th Fibonacci number. When will this function panic?

1. ```
   fn fib(n: u32) -> u32 {
   ```
2. ```
       if n < 2 {
   ```
3. ```
           // The base case.
   ```
4. ```
           todo!("Implement this")
   ```
5. ```
       } else {
   ```
6. ```
           // The recursive case.
   ```
7. ```
           todo!("Implement this")
   ```
8. ```
       }
   ```
9. ```
   }
   ```

11. ```
    fn main() {
    ```
12. ```
        let n = 20;
    ```
13. ```
        println!("fib({n}) = {}", fib(n));
    ```
14. ```
    }
    ```