OK I thought it would be okay just to put the interview question. I didn't expect people to answer it.
But since people post the link to the inorder successor of a Binary SEARCH Tree's link here, I have to say that the problem is not BST but Binary Tree.
The idea is:

If the target node has a right child, then the answer will be the left most child for the right subtree.

If the target node does not have a child, then the answer will be the parent of this node.

If it is the last node in the Inorder Traversal, return null.