[LeetCode] 225. Implement Stack using Queues

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:

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).

利用如下图所示的方式, 每次push的时候将queue里面的值都放到x的后面即可.

Push: O(n), others : O(1)

Code

class MyStack(object):

    def __init__(self):

        """

        Initialize your data structure here.

        """

        self.q = collections.deque()

    def push(self, x):

        """

        Push element x onto stack.

        :type x: int

        :rtype: void

        """

        size = len(self.q)

        self.q.append(x)

        while size:

            self.q.append(self.q.popleft())

            size -= 1

    def pop(self):

        """

        Removes the element on top of the stack and returns that element.

        :rtype: int

        """

        return self.q.popleft()

    def top(self):

        """

        Get the top element.

        :rtype: int

        """

        return self.q[0]

    def empty(self):

        """

        Returns whether the stack is empty.

        :rtype: bool

        """

        return not self.q

# 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()

巴特西

[LeetCode] 225. Implement Stack using Queues_Easy tag: Design

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