LeetCode #938. Range Sum of BST

题目描述：

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:

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

class Solution {
public:
    int rangeSumBST(TreeNode* root, int L, int R) {
        int sum=0;
        DFS(root,L,R,sum);
        return sum;
    }
    
    void DFS(TreeNode* root, int L, int R, int& sum)
    {
        if(root==NULL) return;        
        if(root->val<=R&&root->val>=L) sum+=root->val;
        if(L<root->val) DFS(root->left,L,R,sum);
        if(root->val<R) DFS(root->right,L,R,sum);
    }
};