Description

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Symbol Value
I                 1
V                5
X                10
L                50
C                100
D                500
M                1000

For example, two is written as II in Roman numeral, just two one's added together. Twelve is written as, XII, which is simply X + II. The number twenty seven is written as XXVII, which is XX + V + II.

Roman numerals are usually written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, the number four is written as IV. Because the one is before the five we subtract it making four. The same principle applies to the number nine, which is written as IX. There are six instances where subtraction is used:

I can be placed before V (5) and X (10) to make 4 and 9.
X can be placed before L (50) and C (100) to make 40 and 90.
C can be placed before D (500) and M (1000) to make 400 and 900.
Given a roman numeral, convert it to an integer. Input is guaranteed to be within the range from 1 to 3999.

Example 1:
Input: 3
Output: "III"

Example 2:
Input: 4
Output: "IV"

Example 3:
Input: 9
Output: "IX"

Example 4:
Input: 58
Output: "LVIII"
Explanation: L = 50, V= 5, III = 3.

Example 5:
Input: 1994
Output: "MCMXCIV"
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

AC代码

class Solution {
private:
    vector<pair<int, const char*> > conv = {
        {1000, "M"},
        {900, "CM"},
        {500, "D"},
        {400, "CD"},
        {100, "C"},
        {90, "XC"},
        {50, "L"},
        {40, "XL"},
        {10, "X"},
        {9, "IX"},
        {5, "V"},
        {4, "IV"},
        {1, "I"}
    };
public:
    string intToRoman(int num) {
        string res;
        auto it = conv.begin();
        while((it != conv.end()) && (num > 0)) {
            if(num >= it->first) {
                num -= it->first;
                res += it->second;
            } else {
                ++it;
            }
        }
        return res;
    }
};

测试代码

int main() {
    Solution s;
    int a1 = 3;
    int a2 = 4;
    int a3 = 9;
    int a4 = 58;
    int a5 = 1994;
    cout << a1 << " int to roman -> " << s.intToRoman(a1) << endl;
    cout << a2 << " int to roman -> " << s.intToRoman(a2) << endl;
    cout << a3 << " int to roman -> " << s.intToRoman(a3) << endl;
    cout << a4 << " int to roman -> " << s.intToRoman(a4) << endl;
    cout << a5 << " int to roman -> " << s.intToRoman(a5) << endl;
}

总结

因为罗马数字表示本题的关键就是从大到小排列的，因此可以建立一个pair(类似Python里的字典dict)，如果数字大于当前最大的字符，就在字符串末尾加上一个当前字符。