用c语言写出ackermenn非递归算法

Akerman's non-recursive algorithm是一种用于计算Fibonacci数列的方法，它避免了直接的递归调用，而是通过循环迭代的方式实现。以下是C语言中使用Akerman方法计算Fibonacci数列的一个简单版本：

#include <stdio.h>

// Akerman's non-recursive Fibonacci function
unsigned long long fibonacci(int n) {
    if (n <= 0)
        return 0;
    else if (n == 1)
        return 1;

    unsigned long long fib_n = 1, fib_n_minus_2 = 0, fib_n_minus_1 = 1;
    for (int i = 2; i < n; ++i) {
        unsigned long long temp = fib_n;
        fib_n = fib_n + fib_n_minus_2;
        fib_n_minus_2 = temp;
    }
    return fib_n;
}

int main() {
    int num;
    printf("Enter a number to calculate its Fibonacci: ");
    scanf("%d", &num);

    // Check for valid input range
    if (num < 0) {
        printf("Invalid input! Fibonacci series is not defined for negative numbers.\n");
        return 0;
    }

    unsigned long long result = fibonacci(num);
    printf("The %dth Fibonacci number is: %llu\n", num, result);
    
    // Related questions: