Java [Leetcode 225]Implement Stack using Queues

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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.

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).

解题思路：

代码如下：

class MyStack {
Queue<Integer> queue = new LinkedList<Integer>();
// Push element x onto stack.
public void push(int x) {
queue.offer(x);
} // Removes the element on top of the stack.
public void pop() {
int size = queue.size();
for(int i = 1; i < size; i++)
queue.offer(queue.poll());
queue.poll();
} // Get the top element.
public int top() {
int size = queue.size();
for(int i = 1; i < size; i++)
queue.offer(queue.poll());
int head = queue.poll();
queue.offer(head);
return head;
} // Return whether the stack is empty.
public boolean empty() {
return queue.isEmpty();
}
}