Leetcode: Roman to Integer (36ms) (string)

PROBLEM:

https://leetcode.com/problems/roman-to-integer/


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1. How to read a Roman numbers? Please refer to following link:
http://4thgradecrocs.weebly.com/roman-numerals.html

  I=1   V=5    X=10   L=50   C=100   D=500   M=1000    

2. Basic idea is to read from right to left and :

if left one is >= the right one, use +
if left one is < the right one, use -

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//code 36ms

class Solution {

private:

       int r2n(char c){

              int res;

              switch (c){

              case 'I': res = 1; break;

              case 'V': res = 5; break;

              case 'X': res = 10; break;

              case 'L': res = 50; break;

              case 'C': res = 100; break;

              case 'D': res = 500; break;

              case 'M': res = 1000; break;

              default:;

              }

              return res;

       }

public:

       int romanToInt(string s) {

              int n = s.size(), res = r2n(s[n - 1]);

              //read from right to left

              for (int i = n - 2; i>-1; i--){

                     if (r2n(s[i + 1]) <= r2n(s[i]))res = res + r2n(s[i]);

                     else res = res - r2n(s[i]);

              }

              return res;

       }

};