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:

    3
9       20
    16      7

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: []

Solution

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

from collections import deque
class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        if not root:
            return root
        
        # Add the root to the queue
        queue = deque([root])
        out = []

        while queue:
            level = []
            # Loop through all of the nodes in the
            # level, add each node's value to the
            # level list, and then add each node's 
            # left and right nodes to the queue.
            for _ in range(len(queue)):
                node = queue.popleft()
                level.append(node.val)

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

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

            # Append the level list to the 
            # out list.
            out.append(level)

        return out
        

Leetcode 102 Binary Tree Level Order Traversal