C :

#include <stdio.h>
#include <math.h>
struct list
{
	int left;
	int data;
	char ch;
   int right;
	};
struct list tree[2050];

void print(int root)
{
	if(root!=-1)
	  {
		  print(tree[root].left);
		  print(tree[root].right);
		  printf("%c",tree[root].ch);
		  }
	}
main()
{
	
	int i=1,n;
	//FILE *fin=fopen("fbi.in","r");
	scanf("%d",&n);
	int front,len,node;
	front=pow(2,n);
	len=pow(2,n+1);
	node=pow(2,n-1);
	  
	
	
	for(i=front;i<len;i++)
	 {
		 scanf("%1d",&tree[i].data);
		 tree[i].left=-1;
		 tree[i].right=-1;
		 if(tree[i].data==1)
		   tree[i].ch='I';
		 else
		   tree[i].ch='B';
		 }
		/*for(i=1;i<len;i++)
		 printf("%c ",tree[i].ch);
		 printf("\n");*/
    int temp=n-1;
    while(temp>=0)
      {
	      for(i=pow(2,temp);i<pow(2,temp+1);i++)
	        {
		        tree[i].left=2*i;
		        tree[i].right=2*i+1;
		        if(tree[2*i].ch=='B' && tree[2*i+1].ch=='B')
		        tree[i].ch='B';
		        else if(tree[2*i].ch=='I' && tree[2*i+1].ch=='I')
		        tree[i].ch='I';
		        else tree[i].ch='F';
		        }
	      temp--;
	      }
	   print(1);
	  
	}

C++ :

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int m;
char str[3333];
char put[3] = {'B', 'I' ,'F'};
int dfs(int sx,int ex)
{
    int a,b;
    if(sx==ex)
    {
        printf("%c", put[str[sx] - '0']);
        return str[sx] - '0';
    }
    a=dfs(sx,(sx+ex)/2);
    b=dfs((ex+sx)/2+1,ex);
    if(a == b)
        {
        printf("%c", put[a]);
        return a;
        }
    else
        {
        cout<<'F';
       return 2;
       }
}
int main()
{
    scanf("%d",&m);
    scanf("%s",str);
    dfs(0,strlen(str)-1);
    cout<<endl;
    return 0;
}

Pascal :

var
   n,i,j:longint;
   s:ansistring;
   a:array[0..100]of longint;
   p:array[0..2050,0..2050]of char;
   f:array[0..2050,0..2050]of boolean;
procedure dfs(t,x:longint);
var
   i:longint;
begin
 if t>n+1 then exit;
 for i:=x to a[t] do
  begin
   if p[t,i]=' ' then exit;
   if f[t,i] then continue;
   write(p[t,i]);f[t,i]:=true;
   if i mod 2=0 then
    begin
     if p[t,i]=p[t,i-1]then p[t+1,i div 2]:=p[t,i] else p[t+1,i div 2]:='F';
     dfs(t+1,i div 2);
    end;
  end;
end;
begin
 readln(n);
 a[n+1]:=1;
 for i:=n downto 1 do a[i]:=a[i+1]*2;
 for i:=1 to a[1] do
  for j:=1 to n do p[j,i]:=' ';
 readln(s);
 for i:=1 to a[1] do
  if s[i]='1' then p[1,i]:='I' else p[1,i]:='B';
 dfs(1,1);
end.