Leetcode: 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).
Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

Better Solution: Only push is O(n), others are O(1). Using one queue.

class MyStack {
    // Push element x onto stack.
    Queue<Integer> queue;
    
    public MyStack() {
        this.queue = new LinkedList<Integer>();
    }
    
    public void push(int x) {
        queue.offer(x);
        for (int i=1; i<=queue.size()-1; i++) {
            queue.offer(queue.poll());
        }
    }

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

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

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

就是两个queues, 轮流把除最后一个之外的都放入另一个queue中.

class MyStack {
    // Push element x onto stack.
    Queue<Integer> q1;
    Queue<Integer> q2;
    
    public MyStack() {
        q1 = new LinkedList<Integer>();
        q2 = new LinkedList<Integer>();
    }
    
    public void push(int x) {
        q1.offer(x);
    }

    // Removes the element on top of the stack.
    public void pop() {
        if (q1.isEmpty()) return;
        while (q1.size() > 1) {
            q2.offer(q1.poll());
        }
        q1.poll();
        q1 = q2;
        q2 = new LinkedList<Integer>();
    }

    // Get the top element.
    public int top() {
        if (q1.isEmpty()) return -1;
        while (q1.size() > 1) {
            q2.offer(q1.poll());
        }
        int res = q1.poll();
        q1 = q2;
        q1.offer(res);
        q2 = new LinkedList<Integer>();
        return res;
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return q1.isEmpty();
    }
}