938. 二叉搜索树的范围和

题目

给定二叉搜索树的根结点 root，返回值位于范围 [low, high] 之间的所有结点的值的和。

思路：前序遍历+递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int sum = 0;
    public int rangeSumBST(TreeNode root, int low, int high) {
        if (root == null) return sum;
        if (root.val >= low && root.val <= high) sum += root.val;
        rangeSumBST(root.left, low, high);
        rangeSumBST(root.right, low, high);

        return sum;
    }
}