Convert a base  number to a base  number

 // http://www.careercup.com/question?id=24313662

 #include <iostream>

 #include <string>

 using namespace std;

 string baseConvert2To4(string num)

 {

     string res;

     int n = (int)num.size();

     int i;

     res.resize((n + ) >> );

     for (i = n - ; i >= ; i -= ) {

         res[(i >> ) + (n & )] = ((num[i] - '') << ) + (num[i + ] - '') + '';

     }

     if (n & ) {

         res[] = num[];

     }

     return res;

 }

 int main()

 {

     string num;

     while (cin >> num) {

         cout << baseConvert2To4(num) << endl;

     }

     return ;

 }