【Tree】938. Range Sum of BST

问题描述：

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.

解题思路：

根据二叉搜索树的性质，其中序遍历是升序排列。因此，只需要进行中序遍历，判断当前节点是否在 L 和 R 之间，然后累加 sum 值即可。

Python3 实现：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def rangeSumBST(self, root: TreeNode, L: int, R: int) -> int:
        sum = 0
        if root == None:
            return 0
        sum += self.rangeSumBST(root.left, L, R)
        if L <= root.val <= R:
            sum += root.val
        sum += self.rangeSumBST(root.right, L, R)
        return sum