如何用C++编写一个函数来计算斐波那契数列？

在C++中，我们可以使用递归或循环结构来编写一个计算斐波那契数列的函数。这里分别给出递归和非递归（迭代）两种方法：

### 1. 使用递归

#include <iostream>

// 递归版本的斐波那契函数
int fibonacci_recursive(int n) {
    if (n <= 1)
        return n;
    else
        return fibonacci_recursive(n - 1) + fibonacci_recursive(n - 2);
}

int main() {
    int num;
    std::cout << "Enter a number to calculate Fibonacci: ";
    std::cin >> num;
    
    std::cout << "Fibonacci of " << num << " is: " << fibonacci_recursive(num) << std::endl;

    return 0;
}

### 2. 使用循环

#include <iostream>

// 非递归版本的斐波那契函数
int fibonacci_iterative(int n) {
    if (n <= 1)
        return n;
    int fib_n_minus_1 = 0, fib_n_minus_2 = 1, current_fib = 0;
    for (int i = 2; i <= n; ++i) {
        current_fib = fib_n_minus_1 + fib_n_minus_2;
        fib_n_minus_1 = fib_n_minus_2;
        fib_n_minus_2 = current_fib;
    }
    return current_fib;
}

int main() {
    int num;
    std::cout << "Enter a number to calculate Fibonacci: ";
    std::cin >> num;
    
    std::cout << "Fibonacci of " << num << " is: " << fibonacci_iterative(num) << std::endl;

    return 0;
}