LeetCode-225-Implement Stack using Queues-M（用队列实现堆栈）

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).

思路：
具体做法是每往队列中插入一个元素，都将它前面的元素弹出并重新插入队列中，这样就能保证最后插入队列的元素始终在队列的最前端，比如插入a,b,c,d这四个元素，队列中元素的顺序依次为a,ab,abc,abcd，这样插入的时间复杂度是O(n)，弹出和获取最后一个元素的时间复杂度是O(1)

代码如下：

class MyStack {
 
private:
    queue q;
    int size;
  
public:
    MyStack() {
        size=0;
    }
  
  /** Push element x onto stack. */
    void push(int x) {
       	if(q.size() == 0) {
       	    q.push(x);
   	}
   	else {
  	    q.push(x);
   	    for(int i=0; i