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Implement Queue using Stacks

Implement the following operations of a queue using stacks.

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

分析：

用栈实现队列，需要两个栈来回传输元素。


代码：

class Queue(object):
    def __init__(self):
        """
        initialize your data structure here.
        """
        self.st1 = []
        self.st2 = []

    def push(self, x):
        """
        :type x: int
        :rtype: nothing
        """
        self.move(self.st1, self.st2)
        self.st1.append(x)

    def pop(self):
        """
        :rtype: nothing
        """
        self.move(self.st2, self.st1)
        self.st2.pop()

    def peek(self):
        """
        :rtype: int
        """
        self.move(self.st2, self.st1)
        return self.st2[-1]

    def empty(self):
        """
        :rtype: bool
        """
        if self.st1 or self.st2:
            return False
        return True

    def move(self, st1, st2):
        """
        :Move st2 to st1
        """
        while st2:
            st1.append(st2.pop())

本文标签： implementLeetCodeStacksQueue

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