https://adventofcode.com/2019/day/18

Approach:

1. Reduce maze to a graph with extractGraph Nodes: starting points, gates, keys Edges: shortest direct route between nodes
2. Implement nextKey function to find list of reachable keys for a particular robot.
3. Use Djikstra search to search the space of picking a robot to move from its current position to an unvisited key until all keys are visited.