直接for循环，并且判断是否出现IV等情况

int which(char ch){    if(ch == 'I')        return 1;    else if(ch == 'V')        return 5;    else if(ch == 'X')        return 10;    else if(ch == 'L')        return 50;    else if(ch == 'C')        return 100;    else if(ch == 'D')        return 500;    else return 1000;}class Solution {public:    int romanToInt(string s) {        int flag = 0;        int ans = 0;        for(int i=0; i