Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).


For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7



return its level order traversal as:

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

Source: leetcode | Difficulty: MEDIUM_HARD
Tags: Tree algorithms

Graph traversal

Starter code: # 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: TreeNode) -> List[List[int]]:
        

Solutions: