Implement Graph with BFS and DFS

Implement a graph stored as an adjacency list, with breadth-first and depth-first search.

Implement the Graph class:

• Graph(directed) creates an empty graph. If directed is false, every edge works in both directions.
• addEdge(u, v) adds an edge from u to v (and from v to u when undirected). Vertices are integers and come into existence the first time they appear.
• bfs(start) returns the vertices reachable from start in breadth-first visiting order.
• dfs(start) returns the vertices reachable from start in depth-first preorder.

To make the order unique, always visit a vertex's neighbours in the order its edges were added. start is always included, even when it has no edges or has never appeared in an edge. Each vertex appears at most once.