leetcode 225. Implement Stack using Queues（python）

描述

Implement a last in first out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal queue (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

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, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue), as long as you use only a queue's standard operations.

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False	

Note:

1 <= x <= 9
At most 100 calls will be made to push, pop, top, and empty.
All the calls to pop and top are valid.

解析

根据题意，就是根据题意构造各个函数，思路很简单，都是对列表的基本操作，没什么可解释的。

解答

class MyStack(object):

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

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

    def pop(self):
        """
        Removes the element on top of the stack and returns that element.
        :rtype: int
        """
        if self.stack:
            return self.stack.pop(-1)
        return None

    def top(self):
        """
        Get the top element.
        :rtype: int
        """
        if self.stack:
            return self.stack[-1]
        return None

    def empty(self):
        """
        Returns whether the stack is empty.
        :rtype: bool
        """
        if not self.stack:
            return True
        return False
        
        	      
		

运行结果

Runtime: 16 ms, faster than 68.26% of Python online submissions for Implement Stack using Queues.
Memory Usage: 13.7 MB, less than 13.60% of Python online submissions for Implement Stack using Queues.

原题链接：leetcode.com/problems/im…

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