leetcode-225-implement-stacks-using-queue

Description

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.
Example:
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MyStack stack = new MyStack();

stack.push(1);
stack.push(2);
stack.top(); // returns 2
stack.pop(); // returns 2
stack.empty(); // returns false
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).

思路0

題意可知是用隊列實現棧，那麼聲明一個個隊列。

class MyQueue {
    Queue<Integer> queue;
    
    public MyStack()
    {
        this.queue=new LinkedList<Integer>();
    }
    
    // Push element x onto stack.
    public void push(int x) 
    {
       queue.add(x);
       for(int i=0;i<queue.size()-1;i++)
       {
           queue.add(queue.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();
    }
}