Implement the following operations of a stack using queues.
- push(x) -- Push element x onto stack.
- pop() -- Removes the element on top of the stack.
- top() -- Get the top element.
- empty() -- Return whether the stack is empty.
Notes:
- You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
- Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue -- which means only
push to back,pop from front,size, andis emptyoperations are valid.
Credits:
Special thanks to @jianchao.li.fighter for adding this problem and all test cases.
用队列Queues的基本操作来实现栈Stack，队列和栈都是重要的数据结构，区别主要是，队列是先进先出，而栈是先进后出。
解法1: 在队列push进来新元素时，就把其它元素pop出来排到新元素的后面，新元素就在前面了，就可以后进先出。就好像大家都在排队，来了个重要客人，要让他第一，其他人从前面按顺序跑到他后面。
解法2: push的时候，其他元素不动只是用一个变量记住这个新元素。当top的时候直接给这个变量的值。当pop时，在调整顺序，把最后一个排到前面，弹出。变量记住目前在最尾部的值。
Python: Time: push: O(n), pop: O(1), top: O(1)， Space： O(n)
class Queue:
def __init__(self):
self.data = collections.deque()
def push(self, x):
self.data.append(x)
def peek(self):
return self.data[0]
def pop(self):
return self.data.popleft()
def size(self):
return len(self.data)
def empty(self):
return len(self.data) == 0
class Stack:
# initialize your data structure here.
def __init__(self):
self.q_ = Queue()
# @param x, an integer
# @return nothing
def push(self, x):
self.q_.push(x)
for _ in xrange(self.q_.size() - 1):
self.q_.push(self.q_.pop())
# @return nothing
def pop(self):
self.q_.pop()
# @return an integer
def top(self):
return self.q_.peek()
# @return an boolean
def empty(self):
return self.q_.empty()
Python: Time: push: O(1), pop: O(n), top: O(1)，Space: O(n)
class Stack:
# initialize your data structure here.
def __init__(self):
self.q_ = Queue()
self.top_ = None
# @param x, an integer
# @return nothing
def push(self, x):
self.q_.push(x)
self.top_ = x
# @return nothing
def pop(self):
for _ in xrange(self.q_.size() - 1):
self.top_ = self.q_.pop()
self.q_.push(self.top_)
self.q_.pop()
# @return an integer
def top(self):
return self.top_
# @return an boolean
def empty(self):
return self.q_.empty()
类似题目：
[LeetCode] 232. Implement Queue using Stacks 用栈来实现队列
