#A tree is a binary tree if its inorder traversal is sorted
a = []
def checkBST(root):
        if not root : return True
        def inorder(root):
                if root.left :
                        inorder(root.left)
                a.append(root.data)
                if root.right:
                        inorder(root.right)
        inorder(root)
        return a==sorted(list(set(a)))