为什么下面代码1个AC，1个WA，其他都是RE

#include <iostream>
using namespace std;

string a;
string dfs(string str){
	if(stoi(str) == 0)	return "A";
	if(stoi(str) == 1)	return "B";
	string stra = "";
	for (int i = 1;i <= str.size()/2;i++){
		stra += str[i-1];
	}
	string strb = "";
	for (int i = str.size()/2+1;i <= str.size();i++){
		strb += str[i-1];
	}
	return "C"+dfs(stra) + dfs(strb);
}

int main(){
	cin >> a;
	cout << dfs(a);
	return 0;
}