<p>The data structure <code>TreeNode</code> is used for binary tree, but it can also used to represent a single linked list (where left is null, and right is the next node in the list). Implement a method to convert a binary search tree (implemented with <code>TreeNode</code>) into a single linked list. The values should be kept in order and the operation should be performed in place (that is, on the original data structure).</p>
<p>Return the head node of the linked list after converting.</p>
<p><b>Note: </b>This problem is slightly different from the original one in the book.</p>