LeetCode 232. Implement Queue using Stacks

题目

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

代码

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class MyQueue(object):
def __init__(self):
"""
initialize your data structure here.
"""
self.inStack, self.outStack = [], []
def push(self, x):
"""
:type x: int
:rtype: nothing
"""
self.inStack.append(x)
def pop(self):
"""
:rtype: int
"""
self.move()
return self.outStack.pop()
def peek(self):
"""
:rtype: int
"""
self.move()
return self.outStack[-1]
def empty(self):
"""
:rtype: bool
"""
return (not self.inStack) and (not self.outStack)
def move(self):
"""
:rtype nothing
"""
if not self.outStack:
while self.inStack:
self.outStack.append(self.inStack.pop())
# 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()

点评

可以用两个栈来实现队列,在 pop 的时候,需要迁移栈,利用平均时间复杂度,可以算出来 pop 和 peek 都是 O(1)
注意: 这里题目要求 pop 的时候返回数据