这棵树里有几个黑节点

一棵小树：17 是黑的根，左边挂红色的 15，右边挂黑色的 24，15 的左边还挂着红色的 13。运行下面这段程序：

class RNode:
    def __init__(self, val, color):
        self.val = val
        self.color = color
        self.left = None
        self.right = None

def rb_tree():
    root = RNode(17, "黑")
    root.left = RNode(15, "红")
    root.right = RNode(24, "黑")
    root.left.left = RNode(13, "红")
    return root

def count_black(node):
    if node is None:
        return 0
    n = 1 if node.color == "黑" else 0
    return n + count_black(node.left) + count_black(node.right)

print(count_black(rb_tree()))