Resource
- http://www.thelearningpoint.net/computer-science/algorithms-graph-traversal---breadth-first-search-with-c-program-source-code
- https://en.wikipedia.org/wiki/Breadth-first_search
- https://www.khanacademy.org/computing/computer-science/algorithms/breadth-first-search/a/breadth-first-search-and-its-uses
- http://wenku.baidu.com/link?url=G1jI32NRpkDFyiEzcvEU3BVYcDRkJPXAsmZCl34rpJUBYl2eR6JPo_YdDkrbQHQcEKse75hfCl6OkR7BKAsn4ocOZSd2VFnX-SZfEqST3vG
- https://en.wikipedia.org/wiki/Graph_(discrete_mathematics
- http://nthucad.cs.nthu.edu.tw/~yyliu/personal/nou/04ds/bfs.html
- http://nthucad.cs.nthu.edu.tw/~yyliu/personal/nou/04ds/dfs.html