Medium
Quiz
#107 Binary Tree Level Order Traversal II
APPROACH
Given the root of a binary tree, return a bottom-up collection of node values: starting from the deepest level and working up to the root, listing each level's values left to right.
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: []
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What is the optimal approach for this problem?