二叉搜索树的范围和

/*
 * 思路：
 * 使用Java Stream API和递归进行DFS遍历二叉搜索树，对于每个节点，判断其值是否在给定范围内，
 * 如果在范围内则累加到总和，否则根据其值与范围的比较决定是否继续遍历左子树或右子树。
 */

import java.util.stream.Stream;

public class Solution {
    public int rangeSumBST(TreeNode root, int low, int high) {
        // 使用stream进行DFS并计算总和
        return dfs(root, low, high).mapToInt(Integer::intValue).sum();
    }

    private Stream<Integer> dfs(TreeNode node, int low, int high) {
        if (node == null) {
            return Stream.empty();
        }
        Stream<Integer> current = Stream.empty();
        if (node.val >= low && node.val <= high) {
            current = Stream.of(node.val);
        }
        Stream<Integer> left = dfs(node.left, low, high);
        Stream<Integer> right = dfs(node.right, low, high);
        return Stream.concat(current, Stream.concat(left, right));
    }
}