PAT Advanced 13. Roman to Integer

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.

c++代码，无需判断，我们仅需要用map进行一层映射，之后，进行一层遍历。 如果后一个数字在map上映射比前面一个数字来的大，那么ans就加后面一个数减去前面一个数。并且i自增1 否则，直接直接把这个数加在ans上

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