Leaves On A Binary Search Tree

class Node:
    def __init__(self, val):
        self.left_child = None
        self.right_child = None
        self.data = val

def binary_insert(tree, val):
    # insert val at root if tree is empty
    if tree is None:
        tree = Node(val)
    else:
        # if val is less than current node, 
        # insert val into left subtree
        if (tree.data > val):
            tree.left_child = binary_insert(tree.left_child, val)
        # else, insert val into right subtree
        else:
            tree.right_child = binary_insert(tree.right_child, val)
    return tree

4, 2, 1, 5, 6, 3, 7, 9, 8, 12, 10, 11, 13, 15, 14