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: TreeNode, low: int, high: int) -> int:
        # Base case, below leaf node, return 0
        if not root:
            return 0

        # Store subtree sums
        subtree_sums = self.rangeSumBST(root.left, low, high) + self.rangeSumBST(root.right, low, high)

        # Add the current node's value if the current node is within range
        if root.val >= low and root.val <= high:
            return subtree_sums + root.val

        # Return the range sum of the subtree rooted at the current node
        return subtree_sums