# 938. Range Sum of BST

**<font color=red>难度: Medium</font>**

## 刷题内容

> 原题连接

* https://leetcode.com/problems/range-sum-of-bst/

> 内容描述

```
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
Example 2:

Input: root = [10,5,15,3,7,13,18,1,null,6], L = 6, R = 10
Output: 23
 

Note:

The number of nodes in the tree is at most 10000.
The final answer is guaranteed to be less than 2^31.
```

## 解题方案

> 思路 1
******- 时间复杂度: O(N)******- 空间复杂度: O(1)******


递归

beats 100%

```python
class Solution:
    def rangeSumBST(self, root, L, R):
        """
        :type root: TreeNode
        :type L: int
        :type R: int
        :rtype: int
        """
        def helper(root, L, R):
            if not root:
                return 0
            return helper(root.left, L, R) + helper(root.right, L, R) + (root.val if L <= root.val <= R else 0)
        return helper(root, L, R)
```
































