#0057

Binary Tree Level Order Traversal

MediumDSA25 min20 XP

Problem

Given the root of a binary tree, return a list of levels, where each level is a list of node values from left to right.

Why This Matters

Level-order traversal is the standard tree BFS pattern. It teaches queue processing and how to separate one layer from the next.

Function Signature

def level_order(root):

Examples

Example 1

Inputroot = 3 with children 9 and 20; 20 has children 15 and 7

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

Each inner list contains one horizontal level of the tree.

Constraints

Return [] for an empty tree.
Preserve left-to-right order inside each level.
Use BFS or an equivalent level-aware traversal.

CodePython

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def level_order(root):
    """
    Input example: root = 3 with children 9 and 20; 20 has children 15 and 7
    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 []

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def level_order(root):
    """
    Input example: root = 3 with children 9 and 20; 20 has children 15 and 7
    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.

Testcases1 visible / 3 hidden categories

Three-level tree

Input3 / 9,20 / 15,7

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

The queue processes one level at a time.

Hidden Test Categories

Empty treeOnly left childrenOnly right children