YY第一个编程题本地可以过，为什么考试的通过率为0

#include<iostream>
#include<string>
using namespace std;


char * Fun(char *str,char *des)
{  int i = 0;  int j = 0;  while (str[i] != '\0')  {  if (str[i] != '#')  {  des[j++] = str[i++];  }  else  {  des[j++] = '%';  des[j++] = '2';  des[j++] = '0';  i++;  }  }  des[j] = '\0';  return des;
}

int main()
{  char str[50];  cin >> str;   char des[100];  char *res = Fun(str,des);  cout << res << endl;  return 0;
}