【LeetCode】Roman to Integer

题目描述：

Given a roman numeral, convert it to an integer.

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

思路是对字符串从右往左进行遍历。若s[i]>=s[i-1]，则加上该字符代表的数字，else则减去该数字。

代码如下：

class Solution {
public:
	int romanToInt(string s) {
		unordered_map<char, int> roman;
		vector<char> chars = { 'I', 'V', 'X', 'L', 'C', 'D', 'M' };
		vector<int> nums = { 1, 5, 10, 50, 100, 500, 1000 };
		for (int i = 0; i < chars.size(); i++)
			roman[chars[i]] = nums[i];
		int prev(0), res(0);
		for (int i = s.length(); i >= 0; i--){
			char c = s[i];
			if (roman[c] >= prev)
				res += roman[c];
			else
				res -= roman[c];
			prev = roman[c];
		}
		return res;
	}
};