Evaluate N-ary Expression Tree

1def evaluate(root: 'NNode') -> int:
2    if root.kind == 'num':
3        return root.num
4    vals = [evaluate(child) for child in root.children]
5    if root.kind == 'sum':
6        return sum(vals)
7    if root.kind == 'product':
8        res = 1
9        for v in vals:
10            res *= v
11        return res
12    if root.kind == 'max':
13        return max(vals)
14    return min(vals)