938. [Range Sum of BST] (https://leetcode.com/problems/range-sum-of-bst/)

Easy

Python Solution

class Solution:
    def rangeSumBST(self, root: Optional[TreeNode], low: int, high: int) -> int:
   
        if not root: # if root is null / base case
            return 0 
        if root.val>high: # if root value is greater than high
            return self.rangeSumBST(root.left, low, high)
        if root.val<low:
            return self.rangeSumBST(root.right, low, high)
        
        return root.val + self.rangeSumBST(root.left, low, high) + self.rangeSumBST(root.right, low, high) # it means it is in the range of low and high, so we add it to the sum and call the function for the left and right nodes.

Explanation

Read the comments in the code.