LeetCode——Implement Stack using Queues

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.

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).

用队列来实现一个栈。

思路：用两个队列来模拟栈的功能，当前队列(cur)是用来存放数据的，另一个队列是用来替换当前队列的，这样就能操作队尾元素了。两个队列来回切换，一个用来保存数据，另一个用来操作队尾元素。

PS.Java里不能用泛型数组啊，只好用线性表来代替了。代码看起来又复杂了一些。要不要用C++和Python再来一遍。

class MyStack {

    public List<Queue<Integer>> queue;

    public int cur;

    public MyStack() {

        queue = new ArrayList<Queue<Integer>>();

        queue.add(new LinkedList<Integer>());

        queue.add(new LinkedList<Integer>());

        cur = 0;

    }

    // Push element x onto stack.

    public void push(int x) {

        queue.get(cur).offer(x);

    }

    // Removes the element on top of the stack.

    public void pop() {

        while(queue.get(cur).size() > 1) {

            queue.get(1-cur).offer(queue.get(cur).poll());

        }

        queue.get(cur).poll();

        cur = 1 - cur;

    }

    // Get the top element.

    public int top() {

        while(queue.get(cur).size() > 1) {

            queue.get(1-cur).offer(queue.get(cur).poll());

        }

        int t = queue.get(cur).poll();

        queue.get(1-cur).offer(t);

        cur = 1 - cur;

        return t;

    }

    // Return whether the stack is empty.

    public boolean empty() {

        if(queue.get(cur).isEmpty())

            return true;

        else

            return false;

    }

}

巴特西

LeetCode——Implement Stack using Queues

最新文章

热门文章