Given a binary search tree and a node in it, find the in-order successor of that node in the BST. The successor of a node p is the node with the smallest key greater than p.val.
def inorderSuccessor(self, root: 'TreeNode', p: 'TreeNode') -> 'TreeNode':
result,cur = None,root
while cur:
if cur.val > p.val:
result = cur
cur = cur.left
else:
cur = cur.right
return result