编辑代码

#include <array>
#include <string>
#include <string_view>
#include <utility>

using namespace std::literals;

class Roman {
 private:
  using roman_pair = std::pair<std::string_view, int>;
  static constexpr std::array<roman_pair, 13> lookup{{
    {"M"sv , 1000},
    {"CM"sv, 900},
    {"D"sv , 500},
    {"CD"sv, 400},
    {"C"sv , 100},
    {"XC"sv, 90},
    {"L"sv , 50},
    {"XL"sv, 40},
    {"X"sv , 10},
    {"IX"sv, 9},
    {"V"sv , 5},
    {"IV"sv, 4},
    {"I"sv , 1}
  }};

 public:
  static std::string romanize(int n) noexcept {
    std::string result;
    for (const roman_pair& p : lookup) {
      while (n >= p.second) {
        result += p.first;
        n -= p.second;
      }
    }
    return result;
  }

  static int to_int(std::string_view s) noexcept {
    int result = 0;
    for (const roman_pair& p : lookup) {
      while (s.size() >= p.first.size() &&
             std::equal(p.first.begin(), p.first.end(), s.begin())) {
        result += p.second;
        s.remove_prefix(p.first.size());
      }
    }
    return result;
  }

  static bool is_valid_roman(std::string_view s) noexcept {
    for (const roman_pair& p : lookup) {
      while (s.size() >= p.first.size() &&
             std::equal(p.first.begin(), p.first.end(), s.begin())) {
        s.remove_prefix(p.first.size());
      }
    }
    return s.empty();
  }
};

#include <algorithm>
#include <iostream>

int main() {
  std::cout << "输入你想转换的内容。输入阿拉伯数字转换为罗马数字，输入罗马数字转化为阿拉伯数字，输入Q退出。" << std::endl;
  for (;;) {
    int n;
    std::cin >> n;
    if (std::cin.fail()) {
      std::cin.clear();
      std::string s;
      std::getline(std::cin, s);

      std::transform(s.begin(), s.end(), s.begin(), ::toupper);
      if (s == "Q") break;
      if (!Roman::is_valid_roman(s)) {
        std::cout << "你输入的既不是阿拉伯数字也不是正确的罗马数字。" << std::endl;
        continue;
      }

      std::cout << Roman::to_int(s) << std::endl;
    } else {
      if (n <= 0 || n >= 4000) {
        std::cout << "罗马数字不能表示0、负数和大于3999的数。" << std::endl;
        continue;
      }
      std::cout << Roman::romanize(n) << std::endl;
    }
  }

  return 0;
}