Given a roman numeral, convert it to an integer.

RomanToInteger.cs

public class Solution {
    public int RomanToInt(string s) {
     
        int len = s.Length;
        int num = 0;
        int i=0;
        while(i<len){
            if(i-1>=0 && s[i] == 'V' && s[i-1] == 'I'){
                num = num + 4;
            }
            else if(s[i] == 'V'){
                int j = i;
                int five = 0;
                while(j<len && s[j] == 'V'){
                    five++;
                    j++;
                }
                num = num + 5*five;
                i = j;
                i--;
            }
            else if(i-1>=0 && s[i] == 'X' && s[i-1] == 'I'){
                num = num + 9;
            }
            else if(s[i] == 'X'){
                int j = i;
                int ten = 0;
                while(j<len && s[j] == 'X'){
                    ten++;
                    j++;
                }
                if(j<len && (s[j] == 'L' || s[j] == 'C')){
                    ten--;
                }
                num = num + 10*ten;
                i = j;
                i--;
            }
            else if(i-1>=0 && s[i] == 'L' && s[i-1] == 'X'){
                num = num + 40;
            }
            else if(s[i] == 'L'){
                int j = i;
                int fifty = 0;
                while(j<len && s[j] == 'L'){
                    fifty++;
                    j++;
                }
                num = num + 50*fifty;
                i = j;
                i--;
            }
            else if(i-1>=0 && s[i] == 'C' && s[i-1] == 'X'){
                num = num + 90;
            }
            else if(s[i] == 'C'){
                int j = i;
                int hundred = 0;
                while(j<len && s[j] == 'C'){
                    hundred++;
                    j++;
                }
                if(j<len && (s[j] == 'D' || s[j] == 'M')){
                    hundred--;
                }
                num = num + 100*hundred;
                i = j;
                i--;
            }
            else if(i-1>=0 && s[i] == 'D' && s[i-1] == 'C'){
                num = num + 400;
            }
            else if(s[i] == 'D'){
                int j = i;
                int fivehundred = 0;
                while(j<len && s[j] == 'D'){
                    fivehundred++;
                    j++;
                }
                num = num + 500*fivehundred;
                i = j;
                i--;
            }
            else if(i-1>=0 && s[i] == 'M' && s[i-1] == 'C'){
                num = num + 900;
            }
            else if(s[i] == 'M'){
                int j = i;
                int thousand = 0;
                while(j<len && s[j] == 'M'){
                    thousand++;
                    j++;
                }
                num = num + 1000*thousand;
                i = j;
                i--;
            }
            else{
                int j = i;
                int one = 0;
                while(j<len && s[j] == 'I'){
                    one++;
                    j++;
                }
                if(j<len && (s[j] == 'V' || s[j] == 'X')){
                    one--;
                }
                num = num + one;
                i = j;
                i--;
            }
            i++;
            
        }
        
        return num;
        
    }
}