Implement Binary Tree with Traversals

Implement a binary tree that supports insertion and the four standard traversals.

Implement the BinaryTree class:

• BinaryTree() creates an empty tree.
• insert(val) adds a node in the first free position when filling the tree level by level, left to right (so the tree is always complete). The first value becomes the root, the second its left child, the third its right child, and so on.
• inorder(), preorder() and postorder() return the values as a list.
• levelOrder() returns a list of lists, one per level, from the root down.

An empty tree returns [] from every traversal. Duplicate values are allowed.