search algorithm code

how the DFS code works:

1. To begin, we'll place any of the graph's vertices on top of the stack.
2. The top item in the stack should then be added to the vertex's visited list.
3. Next, make a list of the vertex's nearby nodes. Stack the ones that aren't in the visited list of vertices at the top.
4. Finally, repeat steps 2 and 3 until the stack is depleted.