P2562 [AHOI2002] Kitty猫基因编码

显示CE，说substr尚未声明，用万能头也没用，请大佬帮忙修改```
```cpp
#include <iostream>
#include <string>
using namespace std;
void gene(string s){
    if(s=="0")cout<<'A';
    else if(s=="1")cout<<'B';
    else{
        cout<<'C';
        gene(substr(0,s.length()/2));
        gene(substr(s.length()/2,s.length()/2));
    }
}
int main(){
    string s;
    cin>>s;
    gene(s);
    return 0;
}