DFS/Backtracking/BFS in essence is to abstract the problem into a tree or graph then using brute-force to find the solution.
DFS/Backtracking: recursively traverse a N tree, which is derived from the recursive traversal of binary tree
BFS: is to traverse a graph
Graph traversal algorithm: is multi-way tree traversal algorithms with an added visisted array to prevent infinite loops
BFS很適合解決最短路徑的問題，等於二叉树最小深度. Recursive traversal requires visiting all nodes of the the tree to find the target node, whereas level-order traversal can achieve this without visiting all nodes