【leetcode】Integer to Roman & Roman to Integer（easy）

Roman to Integer

Given a roman numeral, convert it to an integer.

Input is guaranteed to be within the range from 1 to 3999.

思路：首先要学一下罗马数字是怎么表示的，参见百度百科

其实看了上面罗马数字的介绍，可以建一个对应表 把数字和字母对应起来。遇到 I X C且后面字母的数字更大时 减去当前数字，否则加上。

int romanToInt(string s) {
        int num = 0;
        int c[108];
        c['I'] = 1;  c['V'] = 5;   c['X'] = 10;
        c['L'] = 50; c['C'] = 100; c['D'] = 500; c['M'] = 1000;  
        for(int i = 0; i < s.size(); ++i)
        {
            if((s[i] == 'I' || s[i] == 'X' || s[i] == 'C') && (i + 1 < s.size() && c[s[i + 1]] > c[s[i]])) //如果当前是I X C 且后面的数字更大 减去当前数字
                num -= c[s[i]];
            else
                num += c[s[i]];
        }
        return num;
    }

Integer to Roman

Given an integer, convert it to a roman numeral.

Input is guaranteed to be within the range from 1 to 3999.

思路：还是用到查找表。不过凡是有4或者9的都比较特殊，所以也放在表里，之后查表即可。

string intToRoman(int num) {
        vector<pair<int, string>> v;
        v.push_back(make_pair(1000, "M"));
        v.push_back(make_pair(900, "CM"));
        v.push_back(make_pair(500, "D"));
        v.push_back(make_pair(400, "CD"));
        v.push_back(make_pair(100, "C"));
        v.push_back(make_pair(90, "XC"));
        v.push_back(make_pair(50, "L"));
        v.push_back(make_pair(40, "XL"));
        v.push_back(make_pair(10, "X"));
        v.push_back(make_pair(9, "IX"));
        v.push_back(make_pair(5, "V"));
        v.push_back(make_pair(4, "IV"));
        v.push_back(make_pair(1, "I"));
        string roman;
        for(int i = 0; i < v.size(); ++i)
        {
            int n = num / v[i].first;
            if(((i & 0x1) == 1) && n > 0)
            {
                roman += v[i].second;
                num -= v[i].first;
            }
            else if(((i & 0x1) == 0))
            {
                while(n--)
                    roman += v[i].second;
                num = num % v[i].first;
            }
        }
        return roman;
    }

更快的代码，表做的更全，循环都省了：

class Solution {
public:
    const static string THOUS[];
    const static string HUNDS[];
    const static string TENS[];
    const static string ONES[];
    string intToRoman(int num) {
        string result;
        result += THOUS[(int)(num/1000)%10];
        result += HUNDS[(int)(num/100)%10];
        result += TENS[(int)(num/10)%10];
        result += ONES[num%10];
        return result;
    }
};

const string Solution::THOUS[]  = {"","M","MM","MMM"};
const string Solution::HUNDS[]  = {"","C","CC","CCC","CD","D","DC","DCC","DCCC","CM"};
const string Solution::TENS[]   = {"","X","XX","XXX","XL","L","LX","LXX","LXXX","XC"};
const string Solution::ONES[]   = {"","I","II","III","IV","V","VI","VII","VIII","IX"};