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

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.

题目理解：

给定一棵二叉树，可以一个范围L，R，计算二叉树中所有在这个范围内的节点值的和

解题思路：

使用递归方法遍历二叉树，将符合要求的节点值加起来

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