Binary Tree Level Order Traversal

Binary Tree Level Order Traversal is a Medium Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Tree Traversal Patterns (DFS & BFS) pattern, in the Level Order Traversal subpattern.

Problem statement

Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).

Example 1:

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

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

Topics and companies

Topics: Tree, Breadth-First Search, Binary Tree.

Reported in interviews at Bloomberg, Amazon, Facebook, Microsoft, LinkedIn, Adobe, Paypal, Google, Uber, Yahoo.

How to practise Binary Tree Level Order Traversal on Thita.ai

Starter code is provided in C, C#, C++, Go, Java, JavaScript and Python. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in O(N), where N is the number of nodes in the tree. Each node is visited exactly once during both tree construction and level order traversal. time and O(N), for the queue used in BFS (in the worst case, the last level of the tree may have up to N/2 nodes), and for the result vector. space.

Open Binary Tree Level Order Traversal in the code editor, or read the Binary Tree Level Order Traversal editorial for a worked solution with its approach and complexity analysis.

Where Binary Tree Level Order Traversal sits in the DSA pattern sheet

• Tree Traversal Patterns (DFS & BFS) pattern guide
• Level Order Traversal subpattern
• The full DSA patterns sheet
• All coding practice problems

Problems related to Binary Tree Level Order Traversal

Other problems that use the same Level Order Traversal and Tree Traversal Patterns (DFS & BFS) techniques:

• Binary Tree Right Side View — Medium, same subpattern
• Binary Tree Zigzag Level Order Traversal — Medium, same subpattern
• Find Largest Value in Each Tree Row — Medium, same subpattern
• Maximum Level Sum of a Binary Tree — Medium, same subpattern
• All Nodes Distance K in Binary Tree — Medium, same pattern
• Balanced Binary Tree — Easy, same pattern