cloneGraph = function(node, seen = new Map()) {
    const isBaseCase = node === null;
    if (isBaseCase) return null;

    if (seen.has(node)) return seen.get(node);

    return dfs(node, seen);                              /* Time O(V + E) | Space O(N) */
}
https://leetcode.com/problems/clone-graph/
Time O(V + E) | Space O(N)
@param {Node} node
@return {Node}