> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# Binary Tree Level Order Traversal

> Tested Python solution for LeetCode 102 with 13 pytest cases. Generate a practice environment with lcpy.

LeetCode 102, Medium. Topics: Tree, Breadth-First Search, Binary Tree. [View on LeetCode](https://leetcode.com/problems/binary-tree-level-order-traversal/description/).

Generate this problem as a practice environment: tested reference solution, 13 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 102   # by problem number
lcpy gen -s binary_tree_level_order_traversal   # by problem name
```

## Problem

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).

### Examples

![Example 1](https://assets.leetcode.com/uploads/2021/02/19/tree1.jpg)

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

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

```
Input: root = []
Output: []
```

### Constraints

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

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/binary_tree_level_order_traversal/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/binary_tree_level_order_traversal/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
from collections import deque

from leetcode_py import TreeNode


class Solution:
    # Time: O(n)
    # Space: O(w) where w is max width of tree
    def level_order(self, root: TreeNode[int] | None) -> list[list[int]]:
        if not root:
            return []

        result = []
        queue = deque([root])

        while queue:
            level_size = len(queue)
            level = []

            for _ in range(level_size):
                node = queue.popleft()
                level.append(node.val)

                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)

            result.append(level)

        return result
```

## Complexity

| Time | Space                             |
| ---- | --------------------------------- |
| O(n) | O(w) where w is max width of tree |

## Tags

[Grind 75](/catalog/grind-75), [Grind](/catalog/grind), [Blind 75](/catalog/blind-75), [NeetCode 150](/catalog/neetcode-150), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode), [AlgoMaster 75](/catalog/algo-master-75).