implement-stack-using-queues.py

# Time: push: O(n), pop: O(1), top: O(1)
# Space: O(n)
#
# 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.
# 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).

import collections


class Queue:
    def __init__(self):
        self.data = collections.deque()

    def push(self, x):
        self.data.append(x)

    def peek(self):
        return self.data[0]

    def pop(self):
        return self.data.popleft()

    def size(self):
        return len(self.data)

    def empty(self):
        return len(self.data) == 0


class Stack:
    # initialize your data structure here.
    def __init__(self):
        self.q_ = Queue()

    # @param x, an integer
    # @return nothing
    def push(self, x):
        self.q_.push(x)
        for _ in xrange(self.q_.size() - 1):
            self.q_.push(self.q_.pop())

    # @return nothing
    def pop(self):
        self.q_.pop()

    # @return an integer
    def top(self):
        return self.q_.peek()

    # @return an boolean
    def empty(self):
        return self.q_.empty()


# Time: push: O(1), pop: O(n), top: O(1)
# Space: O(n)
class Stack2:
    # initialize your data structure here.
    def __init__(self):
        self.q_ = Queue()
        self.top_ = None

    # @param x, an integer
    # @return nothing
    def push(self, x):
        self.q_.push(x)
        self.top_ = x

    # @return nothing
    def pop(self):
        for _ in xrange(self.q_.size() - 1):
            self.top_ = self.q_.pop()
            self.q_.push(self.top_)
        self.q_.pop()

    # @return an integer
    def top(self):
        return self.top_

    # @return an boolean
    def empty(self):
        return self.q_.empty()