题目地址：https://leetcode.com/problems/implement-stack-using-queues/#/descriptionopen in new window
题目描述
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.
Example:
MyStack stack = new MyStack();
stack.push(1);
stack.push(2);
stack.top(); // returns 2
stack.pop(); // returns 2
stack.empty(); // returns false
Notes:
1、 Youmustuseonlystandardoperationsofaqueue--whichmeansonlypushtoback,peek/popfromfront,size,andisemptyoperationsarevalid.；
2、 Dependingonyourlanguage,queuemaynotbesupportednatively.Youmaysimulateaqueuebyusingalistordeque(double-endedqueue),aslongasyouuseonlystandardoperationsofaqueue.；
3、 Youmayassumethatalloperationsarevalid(forexample,nopoportopoperationswillbecalledonanemptystack).；
题目大意
金庸队列的操作实现一个栈。
解题方法
一个队列就可以解决，不需要两个队列的。
每次新元素进队列的时候，先把当前的数字进入队列，然后把它前面的所有的元素翻转一下，翻到了它的后面。
Java代码如下：
public class MyStack {
Queue<Integer> queue;
/** Initialize your data structure here. */
public MyStack() {
queue = new LinkedList<Integer>();
}
/** Push element x onto stack. */
public void push(int x) {
queue.add(x);
for(int i = 0; i < queue.size() -1; i++){
queue.add(queue.poll());
}
}
/** Removes the element on top of the stack and returns that element. */
public int pop() {
return queue.poll();
}
/** Get the top element. */
public int top() {
return queue.peek();
}
/** Returns whether the stack is empty. */
public boolean empty() {
return queue.isEmpty();
}
}
/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* boolean param_4 = obj.empty();
*/
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Python代码如下：
class MyStack(object):
def __init__(self):
"""
Initialize your data structure here.
"""
self.que = collections.deque()
def push(self, x):
"""
Push element x onto stack.
:type x: int
:rtype: void
"""
self.que.append(x)
for i in range(len(self.que) - 1):
self.que.append(self.que.popleft())
def pop(self):
"""
Removes the element on top of the stack and returns that element.
:rtype: int
"""
return self.que.popleft()
def top(self):
"""
Get the top element.
:rtype: int
"""
return self.que[0]
def empty(self):
"""
Returns whether the stack is empty.
:rtype: bool
"""
return not self.que
# Your MyStack object will be instantiated and called as such:
# obj = MyStack()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.top()
# param_4 = obj.empty()
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