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).

解题要点：

用两个queue，当需要加进一个新元素，先加到q2，放到其头部，然后把q1的元素全部倾倒过来。随后再把q2里的元素倾倒在q1里，这样我们就有了能提取数据就像stack一样的q1。

class MyStack {
    Queue<Integer> q1;
    Queue<Integer> q2;
    int size;
    /** Initialize your data structure here. */
    public MyStack() {
        q1 = new LinkedList<>();
        q2 = new LinkedList<>();
        size = 0;
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        q2.add(x);
        while(q1.size() > 0){
            q2.add(q1.poll());
        }
        while(q2.size() > 0){
            q1.add(q2.poll());
        }
        size++;
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        size--;
        return q1.poll();
    }
    
    /** Get the top element. */
    public int top() {
        return q1.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return size == 0;
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */