README.md

232. 用栈实现队列

English Version

题目描述

使用栈实现队列的下列操作：

• push(x) -- 将一个元素放入队列的尾部。
• pop() -- 从队列首部移除元素。
• peek() -- 返回队列首部的元素。
• empty() -- 返回队列是否为空。

示例:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // 返回 1
queue.pop();   // 返回 1
queue.empty(); // 返回 false

说明:

• 你只能使用标准的栈操作 -- 也就是只有 push to top, peek/pop from top, size, 和 is empty 操作是合法的。
• 你所使用的语言也许不支持栈。你可以使用 list 或者 deque（双端队列）来模拟一个栈，只要是标准的栈操作即可。
• 假设所有操作都是有效的 （例如，一个空的队列不会调用 pop 或者 peek 操作）。

解法

Python3

class MyQueue:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.s1 = []
        self.s2 = []


    def push(self, x: int) -> None:
        """
        Push element x to the back of queue.
        """
        self.s1.append(x)


    def pop(self) -> int:
        """
        Removes the element from in front of queue and returns that element.
        """
        self._move()
        return self.s2.pop()

    def peek(self) -> int:
        """
        Get the front element.
        """
        self._move()
        return self.s2[-1]


    def empty(self) -> bool:
        """
        Returns whether the queue is empty.
        """
        return len(self.s1) + len(self.s2) == 0


    def _move(self):
        """
        Move elements from s1 to s2.
        """
        if len(self.s2) == 0:
            while len(self.s1) > 0:
                self.s2.append(self.s1.pop())


# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()

Java

class MyQueue {

    private Deque<Integer> s1;
    private Deque<Integer> s2;

    /** Initialize your data structure here. */
    public MyQueue() {
        s1 = new ArrayDeque<>();
        s2 = new ArrayDeque<>();
    }

    /** Push element x to the back of queue. */
    public void push(int x) {
        s1.push(x);
    }

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

    /** Get the front element. */
    public int peek() {
        move();
        return s2.peek();
    }

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.isEmpty() && s2.isEmpty();
    }

    /** Move elements from s1 to s2. */
    private void move() {
        if (s2.isEmpty()) {
            while (!s1.isEmpty()) {
                s2.push(s1.pop());
            }
        }
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */

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