Improve Breadth First Search to use the standard libraries.

Update BFS.cpp to use STL instead of the array implementation

Start by inspecting BFS.cpp to identify the current array-based breadth-first search implementation and the standard-library facilities already used nearby. Replace the array implementation with STL usage while preserving BFS behavior; the work is done when BFS.cpp no longer relies on the array implementation and the program still builds and runs correctly.

Written by the indexing model from the issue text.