LeetCode 225. Implement Stack using Queues(使用队列来实现栈)

原题网址:https://leetcode.com/problems/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 backpeek/pop from frontsize, 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:压栈的时候,使用一个临时队列存储最后压栈的元素(以便最先出队),然后将之前队列的内容逐个添加到临时队列,最后将临时队列覆盖回存放栈数据的队列,实现顺序反转。

源代码:

class MyStack {
    private Queue queue = new LinkedList();
    private Queue buf = new LinkedList();
    // Push element x onto stack.
    public void push(int x) {
        buf.add(x);
        while (!queue.isEmpty()) buf.add(queue.remove());
        Queue temp = queue;
        queue = buf;
        buf = temp;
    }

    // Removes the element on top of the stack.
    public void pop() {
        queue.remove();
    }

    // Get the top element.
    public int top() {
        return queue.peek();
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return queue.isEmpty();
    }
}
时间复杂度:压栈O(n),出栈、查看栈顶为O(1),空间复杂度为O(n)

LeetCode 225. Implement Stack using Queues(使用队列来实现栈)_第1张图片



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