Binary Tree Level Order Traversal

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:

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: []

Constraints:

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

 # 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
class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:

        queue, output = [(root, 1)], []

        while queue:
            item, level = queue.pop(0)
            #item, level = data[0], data[1]

            if item:
                if len(output) < level:
                    output.append([item.val])
                else:
                    output[level-1].append(item.val)

                if item.left:
                    queue.append((item.left, level+1))

                if item.right:
                    queue.append((item.right, level+1))
        return output

Binary Tree Level Order Traversal

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