Implementing in C++

How do you implement a "list of lists" in C++? You use std::vector. You use an array of vectors.

vector<int> adj[N]

This creates an array of $N$ separate vectors. Each vector adj[i] stores the neighbors of node $i$. Initially, all vectors are empty. As you read edges from input, you push neighbors into the appropriate vectors. This structure gives you $O(1)$ access to any node's neighbor list and $O(1)$ amortized time to add edges.