Kth ancestor of a node in binary tree

link="https://www.codingninjas.com/codestudio/problems/kth-ancestor-of-a-node-in-binary-tree_842561?leftPanelTab=0"
def findKthAncestor(root, tar, k):
    ans=[None]
    def dfs(x):
        if not x:return None
        if x.data==tar:return 1
        a=dfs(x.left)
        b=dfs(x.right)
        if (a==k or b==k) and not ans[0]:ans[0]=x
        return 1+a if a else (1+b if b else None)
    dfs(root)
    return ans[0].data if ans[0] else -1