232-Implement Queue using Stacks

100-Same Tree | Links:

题意

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty). Implement the MyQueue class:

• void push(int x) Pushes element x to the back of the queue.
• int pop() Removes the element from the front of the queue and returns it.
• int peek() Returns the element at the front of the queue.
• boolean empty() Returns true if the queue is empty, false otherwise. 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, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack’s standard operations.
• Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

Example 1:
Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]

Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

思路

• 用两个栈结构实现queue

class MyQueue(object):
    '''
    stack 先进后出
    queue 先进先出
    用两个stack倒腾一下，实现pop,peek功能
    '''

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.stack1=[]
        self.stack2=[]

    def push(self, x):
        """
        Push element x to the back of queue.
        :type x: int
        :rtype: None
        """
        self.stack1.append(x)

    def pop(self):
        """
        Removes the element from in front of queue and returns that element.
        :rtype: int
        """
        if not self.stack2:
            while self.stack1:
                temp = self.stack1.pop()
                self.stack2.append(temp)
        res = self.stack2.pop()
        return res
                
    def peek(self):
        """
        Get the front element.
        :rtype: int
        """
        if not self.stack2: # stack2为空
            while self.stack1:
                temp = self.stack1.pop()
                self.stack2.append(temp)
        return self.stack2[-1]

    def empty(self):
        """
        Returns whether the queue is empty.
        :rtype: bool
        """
        return not self.stack1 and not self.stack2


# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()