从小到大，记录n-1和n-2的值来求n

    public int Fibonacci(int n) {
        if(n < 2){
            return n;
        }

        int a = 0, b = 1, c = 0;

        for(int i = 2; i <= n; i++){
            c = a + b;
            a = b;
            b = c;
        }

        return c;
    }