13.Roman to Integer (HashTable)

Given a roman numeral, convert it to an integer.

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

class Solution {
public:
    int romanToInt(string s) {
        char c[7] = {'I','V','X','L','C','D','M'};
        int n[7] = {1,5,10,50,100,500,1000};
        unordered_map<char,int> map;
        map.insert(make_pair('I',1));
        map.insert(make_pair('V',5));
        map.insert(make_pair('X',10));
        map.insert(make_pair('L',50));
        map.insert(make_pair('C',100));
        map.insert(make_pair('D',500));
        map.insert(make_pair('M',1000));
        
        int len = s.length();
        int num = 0;
        if(len==1){
            num = map[s[0]];
            return num;
        }
        
        for(int i = 1; i < len; i++){
            if(map[s[i]] <= map[s[i-1]]){
                num += map[s[i-1]];
            }
            else{
                num+= map[s[i]]-map[s[i-1]];
                i++; //dealt two letters at one time, so i should increase 2
            }
        }
        
        //Do not forget to discuss the last case independently
        if(len > 1 && map[s[len-1]] <= map[s[len-2]]){
            num+=map[s[len-1]];
        }
        
        return num;
    }
};

Improve: 使用两个数组代替map，更节省空间。

class Solution {
public:
    int romanToInt(string s) {
        int values[] = {1000, 500, 100, 50, 10, 5, 1 };
        char numerals[] = {'M', 'D', 'C', 'L', 'X', 'V', 'I' };
        int i = 0, j = 0, result = 0;
        while(i < s.length()){
            if(s[i] != numerals[j]){
                j++;
                continue;
            }

            if(i+1<s.length() && j-1>=0 && s[i+1] == numerals[j-1]){
                result += values[j-1]-values[j];
                i+=2;
            }
            else if(i+1<s.length() && j-2>=0 && s[i+1] == numerals[j-2]){
                result += values[j-2]-values[j];
                i+=2;
            }
            else{
                result += values[j];
                i++;
            }
        }
        return result;
    }
};