参考代码:

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


class Solution:
    def levelSum(self, root, level):
        p = []
        self.dfs(root, p, 1, level)
        return sum(p)

    def dfs(self, root, p, dep, level):
        if root is None:
            return
        if dep == level:
            p.append(root.val)
            return
        self.dfs(root.left, p, dep + 1, level)
        self.dfs(root.right, p, dep + 1, level)