- Binary Tree Level Order Traversal
Given the root of a binary tree, return the level order traversal of its nodes’ values. (i.e., from left to right, level by level).
Example 1:
**Input:** root = [3,9,20,null,null,15,7]
**Output:** [[3],[9,20],[15,7]]
Example 2:
**Input:** root = [1]
**Output:** [[1]]
Example 3:
**Input:** root = []
**Output:** []
Constraints:
- The number of nodes in the tree is in the range
[0, 2000]. -1000 <= Node.val <= 1000
Idea
借助于栈，实现BSF