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class TreeNode {
    int val;
    TreeNode left, right;
    TreeNode(int val) {
        this.val = val;
    }
}

public class Solution {
    public int rangeSumBST(TreeNode root, int low, int high) {
        return helper(root, low, high);
    }
    
    private int helper(TreeNode node, int low, int high) {
        if (node == null) return 0;
        
        int sum = 0;
        
        // If the current node's value is within the range, add it to sum
        if (node.val >= low && node.val <= high) {
            sum += node.val;
        }
        
        // If node's value is greater than low, recurse on the left child
        if (node.val < low) {
            sum += helper(node.right, low, high);
        }
        
        // If node's value is less than high, recurse on the right child
        if (node.val > high) {
            sum += helper(node.left, low, high);
        }
        
        return sum;
    }
}