dfs.py

def dfs():
    root = get_root()
    res = float("inf")

    def dfs_search(node, depth):
        if node is not None:
            val = node.val
            if val >= 10:
                nonlocal res
                res = min(res, depth)
            else:
                dfs_search(node.left, depth+1)
                dfs_search(node.right, depth + 1)

    dfs_search(root, 0)
    if res < float("inf"):
        return res
    return -1