Binary Tree Level Order Traversal

Given a binary tree, perform a level order traversal (also known as Breadth-First Search) on the tree. In a level order traversal, you visit the nodes level by level from left to right.

The binary tree is provided in a serialized format as a single line of input in level order where missing nodes are denoted by the token null. For example, the tree

( 1 /
2 3 / \ /
4 5 6 7 )

is given as:

1 2 3 4 5 6 7

If the tree is empty, the input will be an empty string.

Your task is to construct the binary tree from the input and output its level order traversal as a sequence of values separated by a single space.

inputFormat

The input is provided as a single line read from stdin containing the level order serialization of the binary tree. Nodes are separated by spaces. Null nodes are represented by the token null. For example:

1 2 3 4 5 6 7

An empty input indicates an empty tree.

outputFormat

Output the level order traversal of the tree as a sequence of integers separated by a single space. The output should be printed to stdout. For an empty tree, output nothing.

## sample

1 2 3 4 5 6 7

1 2 3 4 5 6 7