225.Implement Stack using Queues (用队列实现栈)

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)

代码：

方法一：使用两个队列

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

方法二：只用一个queue（先push进去，然后把前面的数以此取出，再放入队列）；

    public void push(int x) {
        queue.add(x);
        int len = queue.size()-1;
        while(len>0){
            queue.add(queue.poll());
            len-- ;
        }
    }