基础练习 十六进制转十进制

#include <iostream>
using namespace std;
int change(char x)
{
    if(x>='0'&&x<='9') return x-'0';
    else if(x>='A'&&x<='F') return int(x-'A'+10); 
}
long long fang(long long x)
{
    if(x==1) return 1;
    else if(x>1) return 16*fang(x-1);
}
int main()
{
    string s;
    while(cin>>s){
        int len=s.length();
        int n=1;
        long long sum=0;
        for(int i=len;i--;i>=0){
            //cout<<change(s[i])<<endl;
            sum+=change(s[i])*fang(n);
            n++;
        }    
        cout<<sum<<endl;
    }
    return 0;
}