12. Integer to Roman

Description

Given an integer, convert it to a roman numeral.
Input is guaranteed to be within the range from 1 to 3999.

Solution

• 大->小满减
  设置关键数值及对应字符的map，从大到小遍历，满则减，由于map是按key由小及大排序，因此用rbegin()->rend()保证按key由大及小的遍历顺序

string intToRoman(int num) {
    map myMap = {{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"}};
    string ret = "";
    for (auto it = myMap.rbegin(); it != myMap.rend(); ++it) {
        while (num >= it->first) {
            ret += it->second;
            num -= it->first;
        }
    }
    return ret;
}

• 暴力映射
  一个<3999的整数，直接求千百十个位的数值，对应映射到对应的string

string intToRoman(int num) {
    vector thousands{"", "M", "MM", "MMM"};
    vector hundreds{"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
    vector tens{"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
    vector digits{"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};
    return thousands[num/1000] + hundreds[(num%1000)/100] + tens[(num%100)/10] + digits[num%10];
}