Binary Tree Level Order Traversal
Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
Given binary tree
3 / \ 9 20 / \ 15 7
return its level order traversal as:
[ , [9,20], [15,7] ]
Challenge 1: Using only 1 queue to implement it.
Challenge 2: Use DFS algorithm to do it.
Keep track of the level in DFS and add the node to the cooresponding level.