LeetCode225 Implemet Stack using Queues Java题解

题目：

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 must use only standard operations of a queue -- which means only

push
to back

,

peek/pop from front

,

size

,
and

is empty

operations are valid.
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.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

解答：
用队列实现栈，队列是先进先出，栈是先进后出，所以每一个队列元素进的时候，我们把这个元素后面的所有队列的元素重新出队列再进队列，这样就保证了最后面进入队列的元素可以最先出队列

代码：

class MyStack {

Queue<Integer> queue=new LinkedList<Integer>();
public void push(int x) {
queue.offer(x);
int i=queue.size()-1;
while(i>0)
{
queue.offer(queue.poll());
i--;
}

}

public void pop() {
queue.poll();
}

public int top() {
return queue.peek();
}

public boolean empty() {
return queue.isEmpty();

}
}