109. Binary Tree Level Order Traversal

You're building a tree visualization component that needs to display nodes level by level.

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

Examples

// Tree:     3
//          / \
//         9   20
//            /  \
//           15   7
levelOrder(root); // [[3], [9, 20], [15, 7]]

// Tree: 1
levelOrder(root); // [[1]]

// Tree: null
levelOrder(null); // []