kth element in tree in order traversal

tree.py

###
#         5
#     3       7
#   2   4   6   8
# 1 
###
# n c   k
# 1 0   1
# 2 1   2 1
# 3 2   3 2 1
# 4 3   4 3 2 1
# 5 4   5 4 3 2 1
# 6 5   6 5 4 3 2 1

def in_order_k(tree_node, k):
    if k < 0:
        return k - 1

    if tree_node.left:
        k = in_order_k(tree_node.left, k)

    if k - 1 == 0:
        print tree_node.value
        return k - 1

    if tree_node.right:
        k = in_order_k(tree_node.right, k - 1)

    return k - 1