[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.

Example:

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).

这题目有点水。应该是cc150的原题。不知道意义何在。原题应该是没有top这个东西的。加了top稍微加了一点难度。

原本的做法也很简单，push是照常push的，复杂度O(n)。但是pop就不是照常poll了。如果你的queue里面有n个元素，那么你就poll n - 1 个，那剩下那个自然就是最后那个，符合stack LIFO的原理。poll出去的那些其实就直接push回去就行了。

譬如原来是 1 -> 2 -> 3 -> 4 -> 5。 你poll了四个并且把他们又push进来之后就成了 5 -> 1 -> 2 -> 3 -> 4。头元素5自然就是最后一个。top稍微增加了一点难度，你需要一个临时变量记录一下，譬如说你如果call top，你就做pop那样的操作并且记录一下头元素，如果再call top或者pop，如果有top元素的记录，就不再操作直接返回top。给出代码如下：

    /** Initialize your data structure here. */
    Integer top;
    Queue<Integer> queue;
    public MyStack() {
        queue = new LinkedList<Integer>();
        top = null;
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        if (top != null) {
            queue.add(top);
            top = x;
        } else {
            top = x;
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        int res = top();
        top = null;
        return res;
    }
    
    /** Get the top element. */
    public int top() {
        if (top == null) {
            int popTimes = queue.size() - 1;
            while (popTimes > 0) {
                queue.add(queue.poll());
                popTimes--;
            }
            top = queue.poll();
        }
        
        return top;
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return top == null && queue.isEmpty();
    }