#0238

Binary Tree Level Order Traversal

MediumDSA28 min20 XP

Problem

A tree node is [value, left, right]. Return a list of levels, where each level is a list of values from left to right.

Why This Matters

Level-order traversal is the tree version of BFS and is the gateway to shortest-path and layer-by-layer reasoning.

Function Signature

def level_order(root):

Examples

Example 1

Inputroot = [3, [9, None, None], [20, [15, None, None], [7, None, None]]]

Output[[3], [9, 20], [15, 7]]

Each BFS layer becomes one list in the answer.

Constraints

Return the exact requested value.
Handle empty or small inputs when the prompt allows them.
Prefer the target DSA pattern over brute force when the input can grow.

CodePython

def level_order(root):
    """
    Input example: root = [3, [9, None, None], [20, [15, None, None], [7, None, None]]]
    Return: [[3], [9, 20], [15, 7]]
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return []

def level_order(root):
    """
    Input example: root = [3, [9, None, None], [20, [15, None, None], [7, None, None]]]
    Return: [[3], [9, 20], [15, 7]]
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return []

Visible browser tests run here when available.

Testcases2 visible / 3 hidden categories

Three levels

Input[3, [9, None, None], [20, [15, None, None], [7, None, None]]]

Expected[[3], [9, 20], [15, 7]]

The queue processes nodes by level.

Empty tree

InputNone

Expected[]

No nodes means no levels.

Hidden Test Categories

single nodeleft-heavy treeright-heavy tree