938. Range Sum of BST

class Solution {
    int sum = 0;
    public int rangeSumBST(TreeNode root, int low, int high){
        if(root == null)
            return 0;
        
        if(root.val >= low && root.val <= high)
           sum = sum + root.val;
        rangeSumBST(root.left, low, high) ;
        
        rangeSumBST(root.right, low, high);
        
        return sum;
    }
}