【LeetCode 0102】【BSF】二叉树的层级遍历

102. 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