Fix binary tree inorder and postorder traversals

[Eddiewalsh572]

Fixes #1633

This PR corrects the recursive calls in the BinaryTree traversal methods.

Changes:

• inOrder() now calls inOrder(root)
• inOrder(Node node) now recursively calls inOrder(...) for the left and right children
• postOrder() now calls postOrder(root)
• postOrder(Node node) now recursively calls postOrder(...) for the left and right children

Testing:

• Compiled BinaryTree.java locally with javac

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