Whats the difference?

#1

vector<vector> v
vector v[n]
they both are 2d matrix

#2 
vector< vector< int > > V(N);

That is the declaration of an object V of type vector< vector< int > > , i.e. “vector of int-vectors”, initialized to hold N default-initialized objects…

…i.e., a vector holding N objects of vector< int > – which are, in turn, empty (because “empty” is what default-initialized vectors are).

vector v[N] (which would not even compile). It is also not the same as vector<int> v[N] – that would be an array of N different vector<int> objects.

refer this
https://stackoverflow.com/questions/35478657/what-is-difference-between-vectorint-vn-and-vector-int-v-n#:~:text=vector%20>%20V(N)%3B,default-initialized%20objects…&text=The%20std%3A%3Avector%20(%20std,which%20is%20dynamic%20in%20size.

#3

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