LeetCode #102 - Binary Tree Level Order Traversal: Given a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level). Example Input: root = [3,9,20,null,null,15,7] Example Output: [[3],[9,20],[15,7]] Constraints: The number of nodes in the tree is in the range [0, 2000]. -1000 <= Node.val <= 1000.