You are given an empty binary search tree (BST).

[Step 1] Please successively insert the data pairs containing the following keys 15, 8, 13, 18, 17, 6, 11, 14, 5 into the tree.

Note: In a tree, each step from top to bottom is called as level of a tree. The level count starts with 1 and increments by 1 at each level or step.