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 {

    private Integer curMax = Integer.MIN_VALUE;
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return bool布尔型
     */
    public boolean isValidBST (TreeNode root) {
        // write code here
        return inorder(root);
    }

    private boolean inorder(TreeNode root) {
        if (root == null) {
            return true;
        }

        if (!inorder(root.left)) {
            return false;
        }
        if (root.val < curMax) {
            return false;
        }
        curMax = root.val;
        return inorder(root.right);
    }
}