Level Order Traversal

Given a binary tree, your task is to perform a level-order traversal. In a level-order traversal, the nodes are visited level by level starting from the root, proceeding from left to right within each level.

The tree is provided in a level-order format (i.e., breadth-first order) where each token is either an integer (representing a node's value) or the string null which indicates the absence of a node. An empty tree is represented by the token null.

Your program should output the values of the nodes level by level. Each level's values must be printed on a separate line with the node values separated by a single space.

The level-order traversal can be mathematically described as follows:

\[ \text{result} = [\text{level}_0, \text{level}_1, \dots, \text{level}_k] \]

where each level_i is a sequence of node values collected from left to right at depth i of the tree.

inputFormat

The input consists of a single line containing space-separated tokens representing the binary tree in level-order. Each token is either an integer or the string null indicating a missing node. For example:

3 9 20 null null 15 7

This corresponds to the binary tree:

    3
   / \
  9  20
    /  \
   15   7

An empty tree is represented by the token null.

outputFormat

Print the values of the nodes in level-order traversal. Each level should be printed on a new line, and the values within the same level should be separated by a space.

For example, the output for the input:

3 9 20 null null 15 7

should be:

3
9 20
15 7

## sample

3 9 20 null null 15 7

3
9 20
15 7

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