Range Sum of BST

Problem statement

Given the root node of a binary search tree and two integers low and high, return the sum of values of all nodes with a value in the inclusive range [low, high].

Example 1:

Input: root = [10,5,15,3,7,null,18], low = 7, high = 15Output: 32Explanation: Nodes 7, 10, and 15 are in the range [7, 15]. 7 + 10 + 15 = 32.

Example 2:

Input: root = [10,5,15,3,7,13,18,1,null,6], low = 6, high = 10Output: 23Explanation: Nodes 6, 7, and 10 are in the range [6, 10]. 6 + 7 + 10 = 23.

Constraints:

• The number of nodes in the tree is in the range [1, 2 * 104].
• 1 <= Node.val <= 105
• 1 <= low <= high <= 105
• All Node.val are unique.

My solution

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @param {number} L
 * @param {number} R
 * @return {number}
 */
var rangeSumBST = function(root, L, R) {
    
    const isInBet = (val) => val >= L && val <= R;
    
    const add = (val, sum) => {
        return isInBet(val) ? val + sum : sum;
    };
    
    const preOrder = (node, sum) => {
        if (node === null) {
            return sum;
        }
        
        return add(node.val, sum) + preOrder(node.left, sum) + preOrder(node.right, sum)
    }
    
    return preOrder(root, 0);
};