96. Binary Tree Level Order Traversal II

MediumBFS

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).

Examples

Input: [3,9,20,null,null,15,7]

Output: [[15,7],[9,20],[3]]

Explanation: Levels bottom-up: [15,7] then [9,20] then [3].

Constraints

  • The number of nodes in the tree is in the range [0, 2000].
  • -1000 <= Node.val <= 1000
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