rust编写一个快速斐波那契函数，并计时

使用数组缓存结果，优化了2^N的递归调用。

use std::time::Instant;
use std::io;
fn fib(x: usize) -> usize {
    let mut buffer = [0usize; 256];
    buffer[1] = 1;
    if x>=2 {
        for i in 2..x {
            buffer[i] = buffer[i-1]+buffer[i-2];
        }
    }
    buffer[x-1]+buffer[x-2]
}
fn main() {
    println!("Please input your guess.");
    let mut guess = String::new();
    io::stdin()
        .read_line(&mut guess)
        .expect("Failed to read line");
    let t1 = Instant::now();
    println!("result {} in {:?}", fib(guess.trim().parse().unwrap()), t1.elapsed());
}