leetcode Roman Integer

class Solution {
 public:
  int romanToInt(string s) {
    if (s.length() < 1)
      return 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;
    int i = s.length() - 1;
    int sum = m[s[i--]];
    while (i >= 0) {
      if (m[s[i + 1]] > m[s[i]])
        sum -= m[s[i]];
      else
        sum += m[s[i]];
      --i;
    }
    return sum;
  }
};

class Solution {
 public:
  string intToRoman(int num) {
    // IMPORTANT: Please reset any member data you declared, as
    // the same Solution instance will be reused for each test case.
    string symbol[]={"M","CM","D","CD","C","XC","L","XL","X","IX","V","IV","I"};
    int value[]={1000,900,500,400,100,90,50,40,10,9,5,4,1};
    string res = "";
    int i = 0;
    while (num > 0) {
      if (num >= value[i]) {
        res += symbol[i];
        num -= value[i];
      }
      else
        ++i;
    }
    return res;
  }
};