LeetCode -- Implement Stacks using Queue

Question:

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

Analysis:

问题描述：用队列模仿一个栈。

思路：用两个队列模仿一个栈。每次要pop或者peek时，使用队列倒换一下，剩下最后一个元素单独处理。当且仅当两个队列都为空时，栈才为空。

Answer：

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

    // Removes the element on top of the stack.
    public void pop() {
        if(!q1.isEmpty()) {
                while(q1.size() > 1) {
                    int i = q1.poll();
                    q2.offer(i);
                }
                q1.poll();
        } else {
                while(q2.size() > 1) {
                    int i = q2.poll();
                    q1.offer(i);
                }
                q2.poll();
        }
    }

    // Get the top element.
    public int top() {
        if(!q1.isEmpty()) {
            while(q1.size() > 1) {
                int i = q1.poll();
                q2.offer(i);
            }
            int i = q1.poll();
            q2.offer(i);
            return i;
        } else {
            while(q2.size() > 1) {
                int i = q2.poll();
                q1.offer(i);
            }
            int i = q2.poll();
            q1.offer(i);
            return i;
        }
    }

    // Return whether the stack is empty.
    public boolean empty() {
        if(q1.size() == 0 && q2.size() == 0)
                return true;
        return false;
    }
}