Roman to Integer

此题的关键在于减的情况只有以下几种：IV/IX/XL/XC/CD/CM
因此在count时若出现这六种情况，先全部算作加的情况，最后再减去两倍的多余部分

public class Solution {
    public int romanToInt(String s) {
        int sum = 0;
        if(s.indexOf("IV")!=-1)
            sum = sum -2;
        if(s.indexOf("IX")!=-1)
            sum = sum -2;
        if(s.indexOf("XL")!=-1)
            sum = sum -20;
        if(s.indexOf("XC")!=-1)
            sum = sum -20;
        if(s.indexOf("CD")!=-1)
            sum = sum -200;
        if(s.indexOf("CM")!=-1)
            sum = sum -200;
        for(int i = 0;i < s.length();i++)
        {
            char ch = s.charAt(i);
            if(ch =='I')
                sum = sum+1;
            else if(ch =='V')
                sum = sum+5;
            else if(ch =='X')
                sum = sum+10;
            else if(ch =='L')
                sum = sum+50;
            else if(ch =='C')
                sum = sum+100;
            else if(ch =='D') 
                sum = sum+500;
            else
                sum=sum+1000;
        }
        return sum;
    }
}