Leetcode # 107. Binary Tree Level Order Traversal II

The provided Java code implements a solution for the problem of generating a level-order traversal (also known as breadth-first search) from bottom to top for a binary tree. The solution uses recursion and a hashmap to collect nodes at each level before reversing the final list.

Here's an explanation of how it works:

1. Recursive Function getRestOfTree:

   • This function recursively traverses the tree, collecting all nodes in a hashmap where keys represent levels (starting from 1) and values are lists of node values.
   • For each level, if both left and right subtrees exist, it combines their results. If only one subtree exists, it adds that subtree's result directly.

2. Main Function levelOrderBottom:

   • This function initializes the process by calling getRestOfTree starting from the root node.
   • After collecting all levels in a hashmap, it converts this data into an ArrayList of lists (one list per level).
   • Finally, it reverses the entire list to get the bottom-up order.

Key Points

• HashMap Usage: The hashmap tree stores nodes at each depth. Keys are integer depths starting from 1.
• **Combining

Read the full article at DEV Community

---

Want to create content about this topic? Use Nemati AI tools to generate articles, social posts, and more.