rangeSumBST = function(root, low, high) {
    
    let total = 0;

    const dfs = (node) => {
        if (!node) return;
        if (node.val >= low && node.val <= high) total += node.val;
        dfs(node.left);
        dfs(node.right);
    }
    dfs(root);
    return total;
}
DFS | Recursion
Time O(n)  | Space O(n)
https://leetcode.com/problems/range-sum-of-bst

@param {TreeNode} root
@param {number} low
@param {number} high
@return {number}