Sum of Nodes in BST Range

import java.util.*;

class Node {
    int data;
    Node left, right;

    Node(int val) {
        data = val;
        left = right = null;
    }
}

class Solution {

    ArrayList<Integer> ob = new ArrayList<Integer>();
    int length = 0;

    public int nodeSum(Node root, int l, int r) {
        traversal(root);
        int sum = 0;
        for (int i = 0; i < length; i++) {
            if (ob.get(i) >= l && ob.get(i) <= r)
                sum = sum + ob.get(i);
        }
        return sum;
    }

    void traversal(Node root) {
        if (root == null)
            return;
        traversal(root.left);
        ob.add(root.data);
        length++;
        traversal(root.right);
    }
}