# [Leet Code] Binary Tree Level Order Traversal II

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

Problem:

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

For example:

Given binary tree `[3,9,20,null,null,15,7]`

,

`3`

/ \

9 20

/ \

15 7

return its bottom-up level order traversal as:

`[`

[15,7],

[9,20],

[3]…