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.
Example:
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek(); // returns 1
queue.pop(); // returns 1
queue.empty(); // returns false
Notes:
- You must use only standard operations of a stack -- which means only
push to top
,peek/pop from top
,size
, andis 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).
LeetCode:链接
剑指offer同题:剑指Offer_编程题05:用两个栈实现队列
LeetCode镜像问题:LeetCode225:Implement Stack using Queues
需要两个栈Stack1和Stack2,push的时候直接push进Stack1。pop需要判断Stack1和Stack2中元素的情况,Stack2有值,直接从Stack2 pop,如果没有值,就把Stack1中的元素加入Stack2中,再进行pop。
和剑指offer不一样的是,多了peek和empty的操作。
class MyQueue(object):
def __init__(self):
"""
Initialize your data structure here.
"""
self.queue = []
self.stack = []
def push(self, x):
"""
Push element x to the back of queue.
:type x: int
:rtype: void
"""
self.queue.append(x)
def pop(self):
"""
Removes the element from in front of queue and returns that element.
:rtype: int
"""
# 如果辅助栈为空 就要往里存数
if not self.stack:
while self.queue:
self.stack.append(self.queue.pop())
return self.stack.pop()
def peek(self):
"""
Get the front element.
:rtype: int
"""
if self.stack:
return self.stack[-1]
else:
return self.queue[0]
def empty(self):
"""
Returns whether the queue is empty.
:rtype: bool
"""
if self.stack or self.queue:
return False
else:
return True
# 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()