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]].

``` ## Approach 1. Initialize a queue and start BFS from the root node. 2. For each level, store the nodes in a list and add them to the result list. 3. After processing all levels, reverse the result list. ## Complexity **Time:** O(n) **Space:** O(n) where n is the number of nodes in the tree. ## Trade-offs - Using an iterative approach with a queue avoids deep recursion and potential stack overflow. ```