Function bfs_distances 

pub fn bfs_distances(
    graph: &[Vec<usize>],
    source: usize,
) -> HashMap<usize, usize>

Compute the shortest distance (in number of edges) from source to every vertex reachable from it.

Returns a map from vertex id to distance. Vertices that are not reachable from source are absent. source itself maps to 0.

Panics

Panics if source >= graph.len().