其实很简单

using namespace std;
int b[110];
int main(){
    int n;
    scanf("%d",&n);
    stack<int>c;
    for(int i = 1;i<=n;i++)scanf("%d",&b[i]);
    for(int i = 1,cur = 1;i<=n;i++){
        while(cur<=b[i]){
            c.push(cur);
            printf("A");
            cur++;
        }
        if(c.top() == b[i]){
            c.pop();
            printf("B");
        }
    }
    while(!c.empty()){
        printf("B");
        c.pop();
    }
    return 0;
}