Backend Engineering Mid-Level programming

LeetCode #107 - Binary Tree Level Order Traversal II: Given a binary tree, return the bottom-up level order traversal of its nodes' values. (i.e., from leaves to root). For example, given binary tree [3,9,20,null,null,15,7], return [[15,7],[9,20],[3]]. Input: [3, 9, 20, null, null, 15, 7] Output: [[15,7],[9,20],[3]] Constraints: - The number of nodes in the tree is in the range [0, 2000]. - -1000 <= Node.val <= 1000.

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