leetcode:225. implement stack using queues

题目描述

Implement the following operations of a stack using queues.

• push(x) — Push element x onto stack.
• pop() — Removes the element on top of the stack.
• top() — Get the top element.
• empty() — Return whether the stack is empty.

Example:

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MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

Notes:

• You must use only standard operations of a queue — which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

代码实现

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class :

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

    def push(self, x: int) -> None:
        """
        Push element x onto stack.
        """
        self.stack.append(x)

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

    def top(self) -> int:
        """
        Get the top element.
        """
        return self.stack[-1]

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