Code：

#include <bits/stdc++.h>
using namespace std;
void read(int &x){
	int s=0,w=1;
	char c=getchar();
	while(c<'0'||c>'9'){
		if(c=='-'){
			w=-1;
		}
		c=getchar();
	}
	while(c>='0'&&c<='9'){
		s=s*10+c-'0';
		c=getchar();
	}
	x=s*w;
	return;
}
int n;
char s[100000];
char tree[100000];
void dfs(int x){
	if(x>=(1<<n)){
		if(s[x]==1){
			tree[x]='I';
		}
		else{
			tree[x]='B';
		}
		printf("%c",tree[x]);
		return;
	}
	dfs(x*2);
	dfs(x*2+1);
	if(tree[x*2]==tree[x*2+1]){
		tree[x]=tree[x*2];
	}
	else{
		tree[x]='F';
	}
	printf("%c",tree[x]);
	return;
}
int main(){
	read(n);
	for(int i=pow(2,n);i<=pow(2,n+1)-1;i++){
		s[i]=getchar();
	}
//	scanf("%s",s+1);
	dfs(1);
	return 0;
}

Please help me and I can follow you