[Leetcode] 938. Range Sum of BST

https://leetcode.com/problems/range-sum-of-bst

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def rangeSumBST(self, root: Optional[TreeNode], low: int, high: int) -> int:
        def sum(node):
            if node is None:
                return 0
            if node.val >= low and node.val <= high:
                return node.val + sum(node.left) + sum(node.right)
            else:
                return sum(node.left) + sum(node.right)
        
        return sum(root)