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).

Solution1：Use one queue, like stack

思路： 每次push的时候，都整理一下顺序(插到最前)：通过将前面的都shift到后面(add(poll))

屏幕快照 2017-09-19 下午4.18.37.png

Time Complexity: Push: O(N) others: O(1)
Space Complexity: O(N)

Solution2：Use one queue, like stack

思路： 思路同Solution1，也可以将整理的过程放在pop和top，每次push的时候直接push。pop时(top同思路)：先都整理一下顺序(最后的元素插到最前)：通过将前面的都shift到后面(add(poll))

屏幕快照 2017-09-19 下午4.24.44.png

Time Complexity: pop, top: O(N) others: O(1)
Space Complexity: O(N)

Solution1 Code：

class MyStack {

    //one Queue solution
    private Queue queue = new LinkedList();

    // Push element x onto stack.
    public void push(int x) {
        queue.add(x);
        for(int i = 0; i < queue.size() - 1; i++) { //rotate the queue to make the tail be the head
            queue.add(queue.poll());
        }
    }

    // Removes the element on top of the stack.
    public int pop() {
        return queue.poll();
    }

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

    // Return whether the stack is empty.
    public boolean empty() {
        return queue.isEmpty();
    }
}

Solution2 Code：

class MyStack {

    //one Queue solution
    private Queue queue = new LinkedList();

    // Push element x onto stack.
    public void push(int x) {
        queue.add(x);
    }

    // Removes the element on top of the stack.
    public int pop() {
        for(int i = 0; i < queue.size() - 1; i++) { //rotate the queue to make the tail be the head
            queue.add(queue.poll());
        }
        return queue.poll();
    }

    // Get the top element.
    public int top() {
        int val = pop();
        queue.add(val);
        return val;
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return queue.isEmpty();
    }
}