Leetcode 225 Implement Stack using Queues

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).

类似用stack实现queue的在 

Leetcode 232 Implement Queue using Stacks

class MyStack 
{
   Queue<Integer> queue = new LinkedList<Integer>();//首先new一个queue
    
    public void push(int x) 
    {
       queue.add(x);//stack和queue的不同在于 队首的元素先出 栈顶的元素先出
       for(int i=0;i<queue.size()-1;i++)
       {
           queue.add(queue.poll());//所以要把队尾的元素挪到队首 
       }
    }

    
    public void pop() 
    {
        queue.poll();
    }

    
    public int top() 
    {
        return queue.peek();
    }

    
    public boolean empty() 
    {
        return queue.isEmpty();
    }
}