Range Sum of BST

Given the 

root

 node of a binary search tree, return the sum of values of all nodes with value between 

L

 and 

R

 (inclusive).

The binary search tree is guaranteed to have unique values.

Example 1:

Input: root = [10,5,15,3,7,null,18], L = 7, R = 15
Output: 32
           

思路：Divide and conquer

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int rangeSumBST(TreeNode root, int L, int R) {
        if(root == null) {
            return 0;
        }
        int sum = 0;
        sum += rangeSumBST(root.left, L, R);
        sum += rangeSumBST(root.right, L, R);
        if(L <= root.val && root.val <= R) {
            sum += root.val;
        }
        return sum;
    }
}