带堆优化的贪心，从题意中我们可以得到的最暴力解法就是，每天都搜索整个数组，找出符合条件的题目并且它的经验是最大的，这样找M天就好了。接下来就是如何优化，在每次找的时候我们只要找经验最大的，因为可以用大顶堆组织可以做的题目，每次升级之后加入新的可以做的题目就OK了。-Optimized with a greedy heap, from the meaning of the questions, we can get the most violent solution is that every day, search the entire array, to find qualified experience in the subject and it is the biggest, so look for M-day fine. The next step is how to optimize in each time looking for the experience as long as we find the greatest, because the big top of the heap can be the subject of the organization can do, each time after the upgrade can be done by adding a new subject was OK.