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