中序走出来第一个是几

把 17、24、15、13、23 依次插进一棵空的 BST。运行下面这段程序：

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

def insert(root, val):
    if root is None:
        return BNode(val)
    if val < root.val:
        root.left = insert(root.left, val)
    elif val > root.val:
        root.right = insert(root.right, val)
    return root

def build():
    root = None
    for v in [17, 24, 15, 13, 23]:
        root = insert(root, v)
    return root

def mid(node, out):
    if node is None:
        return
    mid(node.left, out)
    out.append(node.val)
    mid(node.right, out)

out = []
mid(build(), out)
print(out[0])