问题标题: 酷町堂：快速进制转换

#include <iostream>
using namespace std;
void 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 ;
    cout<<res;
}
int main()
{
    string s;
    cin>>s;
    transform(2,16,s);
}

哪里错了