README.md

225. 用队列实现栈

English Version

题目描述

使用队列实现栈的下列操作：

• push(x) -- 元素 x 入栈
• pop() -- 移除栈顶元素
• top() -- 获取栈顶元素
• empty() -- 返回栈是否为空

注意:

• 你只能使用队列的基本操作-- 也就是 push to back, peek/pop from front, size, 和 is empty 这些操作是合法的。
• 你所使用的语言也许不支持队列。 你可以使用 list 或者 deque（双端队列）来模拟一个队列 , 只要是标准的队列操作即可。
• 你可以假设所有操作都是有效的（例如, 对一个空的栈不会调用 pop 或者 top 操作）。

解法

Python3

class MyStack:

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


    def push(self, x: int) -> None:
        """
        Push element x onto stack.
        """
        self.q.append(x)
        n = len(self.q)
        for i in range(1, n):
            self.q.append(self.q.pop(0))


    def pop(self) -> int:
        """
        Removes the element on top of the stack and returns that element.
        """
        return self.q.pop(0)


    def top(self) -> int:
        """
        Get the top element.
        """
        return self.q[0]


    def empty(self) -> bool:
        """
        Returns whether the stack is empty.
        """
        return len(self.q) == 0



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

Java

class MyStack {

    private Deque<Integer> q;

    /** Initialize your data structure here. */
    public MyStack() {
        q = new ArrayDeque<>();
    }

    /** Push element x onto stack. */
    public void push(int x) {
        q.offerLast(x);
        int n = q.size();
        while (n-- > 1) {
            q.offerLast(q.pollFirst());
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return q.pollFirst();
    }

    /** Get the top element. */
    public int top() {
        return q.peekFirst();
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return q.isEmpty();
    }
}

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

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