Vector of Arrays or Array of vectors cpp

vector<int> adj[n];

Creates an array of n vectors of ints. However, if n is not a compile time constant, this is not standard C++, which does not support variable length arrays. Some compilers may implement it as an extension.

vector<vector <int>> vect;

This creates a vector of vectors of ints.

The dimensions of the latter are no longer fixed, which makes them functionally quite different. vect can contain any number of vector<int> values.

There are also significant ramifications to using std::vector or std::array vs. raw arrays when it comes to passing results to/from functions.