Java [Leetcode 225]Implement Stack using Queues

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();
    }
}