Binary Tree Level Order Traversal

You are given the level order representation of a binary tree as a single line of input. The tree nodes are given in a comma-separated format, where each value represents a node of the tree in level order. If a node does not exist, it is represented by the string null (case insensitive).

Your task is to perform a level order traversal (also known as breadth-first search) of the binary tree and output the values of the nodes level by level.

Note: Each level should be printed on a new line, and the node values within each level should be space-separated.

Example:

Input: 3,9,20,null,null,15,7

The tree corresponds to:
      3
     / \
    9  20
       / \
      15  7

Output:
3
9 20
15 7

Hint: You may use a queue to help traverse the tree level by level.

For more details, recall that the binary tree structure is defined as follows:

$$\textbf{TreeNode}: \quad \{ val,\, left,\, right \} $$

inputFormat

The input is given in a single line as a comma-separated list of values representing the binary tree in level order. Each value is either an integer or the string null (indicating a missing node). For example:

3,9,20,null,null,15,7

outputFormat

Output the level order traversal of the tree. Each level's nodes should be printed on a new line, and the node values within the same level should be separated by a space. No extra spaces or blank lines should be printed.

## sample

3,9,20,null,null,15,7

3
9 20
15 7

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