1 #include 
     
       2 #define _for(i,a,b) for(int i = (a);i < b;i ++) 3 typedef long long ll; 4 using namespace std; 5 string S; 6 int N; 7 inline ll read() 8 { 9     ll ans = 0;10     char ch = getchar(), last = ' ';11     while(!isdigit(ch)) last = ch, ch = getchar();12     while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - '0', ch = getchar();13     if(last == '-') ans = -ans;14     return ans;15 }16 inline void write(ll x)17 {18     if(x < 0) x = -x, putchar('-');19     if(x >= 10) write(x / 10);20     putchar(x % 10 + '0');21 }22 struct TreeNode23 {24     char val;25     TreeNode *left;26     TreeNode *right;27 };28 char judge(int le,int ri)29 {30     bool oflag = false;31     bool zflag = false;32     _for(i,le,ri+1)33     {34         if(S[i]=='0')35             zflag = true;36         else if(S[i]=='1')37             oflag = true;38     }39     if(zflag && !oflag)40         return 'B';41     else if(!zflag && oflag)42         return 'I';43     else44         return 'F';45     return -1;46 }47 TreeNode* build(int le,int ri)48 {49     TreeNode* tmp = (TreeNode*)malloc(sizeof(TreeNode));50     int mi = (le+ri)/2;51     tmp->val = judge(le,ri);52     if(le!=ri)53     {54         tmp->left = build(le,mi);55         tmp->right = build(mi+1,ri);56     }57     else58         tmp->left = tmp->right = NULL;59     return tmp;60 }61 void postorder(TreeNode *root)62 {63     if(!root)64         return ;65     postorder(root->left);66     postorder(root->right);67     printf("%c",root->val);68 }69 int main()70 {71     TreeNode* root;72     scanf("%d",&N);73     cin >> S;74     root = build(0,pow(2,N)-1);75     postorder(root);76     return 0;77 }