Rust Platform

Fibonacci Sequence

Implement the fibonacci function that takes an integer n as input and returns the nth Fibonacci number.

The Fibonacci sequence is a series of numbers where each number (after the first two) is the sum of the two preceding ones. The sequence starts with 0 and 1. For example, the sequence begins: 0, 1, 1, 2, 3, 5, 8, ...

If the input is a negative number, the function should handle it by panicking with a meaningful error message.

1. Given a valid input n = 5, the function should return 5:

   let result = fibonacci(5);  // 5

2. Given another valid input n = 10, the function should return 55:

   let result = fibonacci(10);  // 55

3. If the input is a negative number, such as n = -1, the function should panic with an error message:

   fibonacci(-1);  // Should panic with "Negative input is not allowed"