0107. Binary Tree Level Order Traversal II

https://leetcode.com/problems/binary-tree-level-order-traversal-ii

Description

Given the root of a binary tree, return the bottom-up level order traversal of its nodes' values. (i.e., from left to right, level by level from leaf to root).

Example 1:

**Input:** root = [3,9,20,null,null,15,7]
**Output:** [[15,7],[9,20],[3]]

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

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