用集合模拟栈和队列

package practice0730;
import java.util.ArrayList;
public class Mstack {
    ArrayList<Integer> arr=new ArrayList<Integer>();
    int top=-1;
    public void push(Integer integer){
        arr.add(top, integer);
        top++;
    }
    public Integer pop() {
        top--;
        int r=0;
        try {
            r=arr.get(top );
            arr.remove(top);
        } catch (Exception e) {
            System.out.println("栈已为空");
            return -1;
        }
        return r;
    }
    public boolean isEmpty() {
        if(top<=0){
            return true;
        }else{
            return false;
        }
    }
}