class Solution {
    public int romanToInt(String s) {
        int ans = 0;
        for(int i = 0;i < s.length();i++){
            if(i == s.length()-1 || getVal(s.charAt(i)) >= getVal(s.charAt(i+1))){
                ans += getVal(s.charAt(i));
            }else{
                ans += getVal(s.charAt(i+1))-getVal(s.charAt(i));
                i++;
            }
        }
        return ans;
    }

    public int getVal(char c){
        switch(c){
            case 'I': return 1;
            case 'V': return 5;
            case 'X': return 10;
            case 'L': return 50;
            case 'C': return 100;
            case 'D': return 500;
            case 'M': return 1000;
            default : return 0;
        }

    }
}

Q.E.D.