比火车那题简单

#include<iostream>
#include<stack>
using namespace std;
const int N=1e7+7;
int n,c[N],t=1;
stack<int>s;

int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>c[i];
		cout<<"A";
		s.push(i);
		while(!s.empty()&&s.top()==c[t]&&t<=i){
			cout<<"B";
			s.pop();
			t++;
		}
	}
	while(!s.empty()){
		s.pop();
		cout<<"B";
	}
	return 0;
}