Should be a hard problem.
Core Idea : Fix one number, reduce the problem size, repeat until only two numbers are left, then solve that directly

v.end() : 
A member function
Exists only if v is a container class

end(v) : 
A free function (std::end) which internally calls v.end(), safer, modern and generic

Main difference:
```
int arr[] = {1,2,3};

auto a = end(arr);   //  works
auto b = arr.end(); //  ERROR

```