I am a student who just started to learn programming with c++. So kindly excuse me if my question is full of nonsense. Thank you. So I wonder what is that line "multiset<int> A(int Arr, intArrr+arrSize);" meaning?
Multiset is a container that can hold any type of objects, so multiset<int> just tells you that you store ints.
In this piece of code, you are using the range constructor for A. You need to give it two values: the iterator to the first element, and the iterator to the last element. For arrays (intArr), intArr points to the first element. intArr+arrSize is just simple pointer arithmetic to indicate the last element in the array. Note that on your line 3 you should have intArr not int Arr