Graph algorithms - minimum cost path

Algorithms for finding the minimum cost path between two given vertices.

Example with positive costs:

Example with some negative costs:

Issues:

1. Execute Dijkstra's algorithm on the two examples above;
2. Implement Dijkstra's algorithm (start from BFS from last time and replace the FIFO queue with a priority queue);
3. Analyse its behavior on the two examples above;

Sample partial implementations: parse.py, dijkstra.py and bellman.py