Is Binary Tree Level Order Traversal II easy or hard?

Binary Tree Level Order Traversal II is generally classified as a Medium difficulty problem. The traversal itself is straightforward BFS, but recognizing that the result must be reversed or stacked adds a small twist compared to the standard level order traversal.

How to solve Binary Tree Level Order Traversal II in O(n)?

Use BFS with a queue to traverse the tree level by level. For each level, collect node values and store them in a list. After traversal, reverse the list of levels or use a stack to build the result in bottom-up order. Each node is processed once, giving O(n) time complexity.

Binary Tree Level Order Traversal II Python or Java solution

Python and Java implementations typically use a queue (such as collections.deque in Python or LinkedList in Java) to perform BFS. Each level is processed using the current queue size, and results are reversed or inserted at the front to produce the bottom-up order.

What is the best approach for Binary Tree Level Order Traversal II?

The most common solution uses Breadth-First Search (BFS) with a queue to process nodes level by level. Each level is stored and then reversed at the end, or pushed into a stack while traversing. This approach visits each node once and runs in O(n) time with O(n) space.

What data structure is used in Binary Tree Level Order Traversal II?

The primary data structure is a queue for Breadth-First Search traversal. A stack or list reversal is also used to produce the bottom-up ordering of levels. In recursive implementations, the call stack and a dynamic list of levels store intermediate results.

What is the time complexity of Binary Tree Level Order Traversal II?

Binary Tree Level Order Traversal II runs in O(n) time where n is the number of nodes in the tree. Every node is visited exactly once during traversal. Additional storage for levels, queue, or stack leads to O(n) space complexity.

Is Binary Tree Level Order Traversal II asked at Google, Amazon, or Meta?

Binary tree traversal problems are common in interviews at companies like Amazon, Google, and Meta. Variants of level order traversal frequently appear because they test BFS fundamentals, tree data structures, and queue usage.

107. Binary Tree Level Order Traversal II - Practice Online

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).

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[15,7],[9,20],[3]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

Constraints:

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