Sum Tree

def fun(self, node, ans):
    if not node:
        return 0
    # leaf node
    if not node.left and not node.right:
        return node.data

    # subtree sums
    left_sum = self.fun(node.left, ans)
    right_sum = self.fun(node.right, ans)

    # Sum Tree condition
    if left_sum + right_sum != node.data:
        ans[0] = False

    # total subtree sum
    return left_sum + right_sum + node.data

class Solution:
    def is_sum_tree(self, node):
        if not node:
            return True
        ans = [True]
        self.fun(node, ans)
        return ans[0]