[LeetCode-JAVA] Implement Stack using Queues

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).

题意：用queue来模拟stack，用queue的基本操作

思路：比较简单，用两个queue来进行即可，一个作为模拟的stack存储，一个运来转换中间的操作数字

代码：

class MyStack {
    LinkedList<Integer> queue = new LinkedList<Integer>();
    LinkedList<Integer> temp = new LinkedList<Integer>();
    // Push element x onto stack.
    public void push(int x) {
        while(!queue.isEmpty()){
            temp.offer(queue.poll());
        }
        queue.offer(x);
        while(!temp.isEmpty()){
            queue.offer(temp.poll());
        }
    }

    // Removes the element on top of the stack.
    public void pop() {
        queue.poll();
    }

    // Get the top element.
    public int top() {
        return queue.peek();
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return queue.isEmpty();
    }
}