1. If the root is null, create a new node with the input data and return that node.
2. Else if the input data is less than the data in the root node, recurse through the left sub tree, root.left = insert(root.left, data);
3. Else if the input data is greater than the data in the root node, recurse through the right sub tree, root.right = insert(root.right, data);
4. Return root.
