#include <iostream> int main() { int n; std::cout << "请输入一个整数n(0<=n<=20): "; std::cin >> n; if (n < 0) { std::cout << "负数没有阶乘" << std::endl; return 1; } unsigned long long result = 1; for (int i = 1; i <= n; ++i) { result *= i; } std::cout << n << "!的值为: " << result << std::endl; return 0; }