Leecode no.232 栈实现队列

package leecode;

import java.util.Stack;

/**
 * 两个栈实现队列
 * @CreateDate 2021/4/20
 *
 * 请你仅使用两个栈实现先入先出队列。队列应当支持一般队列支持的所有操作（push、pop、peek、empty）：
 */
public class StackToQueue {
    //标志位 1代表数据在s1,且是正序  2代表数据在s2,且是反序
    Integer flag = 1;
    Stack<Integer> s1 = new Stack();
    Stack<Integer> s2 = new Stack();

    public StackToQueue(){

    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        if(flag == 2){
            while(s2.size() > 0){
                s1.push(s2.pop());
            }
            flag = 1;
        }
        s1.push(x);
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        int r1 = peek();
        int result =  s2.pop();
        return result;
    }

    /** Get the front element. */
    public int peek() {
        if(flag == 1){
            while(s1.size() > 0){
                s2.push(s1.pop());
            }
            flag = 2;
        }
        return s2.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return flag == 1 ? s1.size() == 0 : s2.size() == 0;
    }

    public static void main(String[] args) {
        StackToQueue stackToQueue = new StackToQueue();
        stackToQueue.push(1);
        stackToQueue.push(2);
        System.out.println(stackToQueue.peek());
        System.out.println( stackToQueue.pop());
        System.out.println(stackToQueue.empty());

    }

}