［LeetCode］232. Implement Queue using Stacks

232. Implement Queue using Stacks

思路：用两个栈来回换就行。

class MyQueue(object):
    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.stack = []
        self.stack_temp = []
    def push(self, x):
        """
        Push element x to the back of queue.
        :type x: int
        :rtype: void
        """
        if self.stack_temp:
            while self.stack_temp:
                self.stack.append(self.stack_temp.pop())
        self.stack.append(x)
    def pop(self):
        """
        Removes the element from in front of queue and returns that element.
        :rtype: int
        """
        while self.stack:
            self.stack_temp.append(self.stack.pop())
        if self.stack_temp:
            return self.stack_temp.pop()
        return None
    def peek(self):
        """
        Get the front element.
        :rtype: int
        """
        if self.stack_temp:
            return self.stack_temp[len(self.stack_temp)-1]
        elif self.stack:
            while self.stack:
                self.stack_temp.append(self.stack.pop())
            if self.stack_temp:
                return self.stack_temp[len(self.stack)-1]
        return None
    def empty(self):
        """
        Returns whether the queue is empty.
        :rtype: bool
        """
        return False if self.stack or self.stack_temp else True