difference btw v.begin(),v.cbegin();
Vectors in cpp begin
begin will return an iterator or a const_iterator depending on the const-qualification of the object it is called on.
cbegin will return a const_iterator unconditionally
vec.begin(); //iterator
vec.cbegin(); //const_iterator
const_vec.begin(); //const_iterator
const_vec.cbegin(); //const_iterator