Roman to Integer

题目：
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: “III”
Output: 3

Example 2:

Input: “IV”
Output: 4

Example 3:

Input: “IX”
Output: 9

Example 4:

Input: “LVIII”
Output: 58
Explanation: L = 50, V= 5, III = 3.

Example 5:

Input: “MCMXCIV”
Output: 1994
Explanation: M = 1000, CM = 900, XC = 90 and IV = 4.

注意：将罗马数字转换为对应的阿拉伯数字，可以发现有两条规律①当前数字为最后一个数字或者只有一个数字时，加；②当前数字比下一个数字大时，加③其他情况的话，减。使用哈希表实现。

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