x进制转化为y进制

string transform(int x,int y,string s){    string res = "";    int sum = 0;    for(int i = 0 ;i < s.length(); i++){        if(s[i]=='-')            continue ;        if(s[i]>='0'&&s[i]<='9'){            sum = sum*x+s[i]-'0';        }        else{            sum = sum*x+s[i]-'A'+10;        }    }    while(sum){        char tmp = sum % y;        sum/=y;        if(tmp<=9)            tmp+='0';        else{            tmp = tmp - 10 +'A';        }        res = tmp + res;    }    if(res.length()==0)        res = "0";    if(s[0]=='-')        res ='-' + res ;    return res;}