LeetCode225 Implemet Stack using Queues Java题解

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 emptyoperations 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).

解答：

用队列实现栈，队列是先进先出，栈是先进后出，所以每一个队列元素进的时候，我们把这个元素后面的所有队列的元素重新出队列再进队列，这样就保证了最后面进入队列的元素可以最先出队列

代码：

class MyStack {

	Queue<Integer> queue=new LinkedList<Integer>();
    public void push(int x) {
    	queue.offer(x);
    	int i=queue.size()-1;
    	while(i>0)
    	{
    		queue.offer(queue.poll());
    		i--;
    	}

    }

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

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

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

    }
}

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