#include<bits/stdc++.h>
#define ll long long
#pragma GCC optimize(2)
#define un unsigned
#define int long long
#define db double
#define inf 1000000000000000
#define st string
#define ct const
#define xh(a,b,c) for(int a=b;a<=c;a++)
#define wx while(1)
#define dn(a,b,c) for(int a=b;a>=c;a--)
using namespace std;
string s;
void dfs(string s){
	bool f1=0,f2=0;
	xh(i,0,s.size()-1){
		if(s[i]!='0')f1=1;
		if(s[i]!='1')f2=1;
	}
	if(!f1){
		cout<<'A';
		return ;
	}
	if(!f2){
		cout<<'B';
		return ;
	}
	cout<<'C';
	string s1,s2;
	xh(i,0,s.size()/2-1)s1+=s[i];
	dfs(s1);
	xh(i,s.size()/2,s.size()-1)s2+=s[i];
	dfs(s2);
}
signed main(){
	cin>>s;
	dfs(s);
	return 0;
}

和FBI树很像

#include<bits/stdc++.h>
using namespace std;
int n;
string a;
char dfs(int h,int t)
{
	//cout<<k<<endl;
	if(t-h>=1)
	{
		int s=0,k=0;
		for(int i=h;i<=t;i++)
		s=s+a[i]-48,k++;
		if(s==0)cout<<"A";
		else if(s==k)cout<<"B";
		else
		{
			cout<<"C";
			dfs(h,(t-h)/2+h);
			dfs((t-h)/2+h+1,t);
		}
	}
	else
	{
		if(a[h]=='1')cout<<"B";
		else cout<<"A";
	}
}
int main()
{
	cin>>a;
	dfs(0,a.size()-1);
}