A problem about 315. Count of Smaller Numbers After Self

Hi, there.

In the Binary Index Tree implementation, the length of the tree should be one greater than the length of the original array (` nums `) . However,  your code is the same .

```c++
// @ 63-64 line
vector <int> f(sorted.size());
fenwick = f;
```

I think this will lead to some memory overflow issues. So, I modifies the size of `f` to `sorted.size() + 1`. It works in leetcode.
