Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its node values. In level order traversal, nodes are visited level-by-level from left to right.

The tree is provided as a string representation in level order with missing nodes denoted by null. For example, the tree

    3
   / \
  9  20
    /  \
   15   7

is represented as [3,9,20,null,null,15,7].

Your task is to construct the tree from the input, perform a level order traversal, and output the traversal as a nested list. The output for the above example is [[3],[9,20],[15,7]].

inputFormat

Input is given in a single line through standard input. The line contains a string representation of a binary tree in level order enclosed in square brackets. Nodes are separated by commas, and a missing node is represented by the string null. For example: [3,9,20,null,null,15,7].

outputFormat

Output the level order traversal of the tree as a nested list printed to standard output. For example, for the input [3,9,20,null,null,15,7], the output should be [[3],[9,20],[15,7]].

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