Return the level order traversal of a binary tree's node values (left to right, level by level).

The tree is represented as a nested object: { val: 3, left: { val: 9, left: null, right: null }, right: { val: 20, left: null, right: null } }

Example:

Input: root = [3,9,20,null,null,15,7]
        3
       / \
      9  20
        /  \
       15   7
Output: [[3],[9,20],[15,7]]