1. 有两个栈的队列：用两个栈来实现一个队列，从而使每个队列操作（enqueue(), dequeue()...）使用一个平均情况下的O(C)操作内完成。
English version: Implement a queue with two stacks so that each queue operations takes a constant amortized number of stack operations.
解题思想：
若你将元素全部入栈后再全部出栈，那么他们将以相反的顺序出现。若重复该操作，则他们又以原来的顺序出现了。
If you push elements onto a stack and then pop them all, they appear in reverse order. If you repeat this process, they're now back in order.
2. Stack with max. Create a data structure that efficiently supports the stack operations( push and pop) and also a return-the-maximum operation. Assume the elements are real numbers so that you can compare them.
Hint: Use two stacks, one to store all of the items and a second stack to store the maximums.