快排 quick sort

实现快速排序，Implement Quicksort and answer the following questions. (1) How many comparisons will Quicksort do on a list of n elements that all have the same value? (2) What are the maximum and minimum number of comparisons will Quicksort do on a list of n elements, give an instance for maximum and minimum case respectively.

优先级队列 priority queue

优先级队列的实现，包括堆的实现，最大堆的生成

给n个整数的集合s和一个整数x，判断是否存在两个数的和为x

算法课本的题目，要求复杂度是(nlgn)。