题解 | #判断是不是二叉搜索树#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, null, null);
    }

    private boolean isValidBST(TreeNode node, Integer min, Integer max) {
        if (node == null) {
            return true;
        }
        if ((min != null && node.val <= min) || (max != null && node.val >= max)) {
            return false;
        }
        return isValidBST(node.left, min, node.val) &&
               isValidBST(node.right, node.val, max);
    }
}