LeetCode 938.

package lc_938;

public class Solution {

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