98_validate_binary_search_tree.java

/**
 * 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 {
    public boolean isValidBST(TreeNode root) {
        ArrayList<Integer> arr = new ArrayList<>();
        inOrderTraversal(root, arr);
        boolean ans = true;
        for (int i = 0; i < arr.size() - 1; i++) {
            if (arr.get(i) >= arr.get(i+1)) {
                ans = false;
                break;
            }
        }
        return ans;
    }
    
    static void inOrderTraversal(TreeNode root, ArrayList<Integer> arr) {
        if (root != null) {
            inOrderTraversal(root.left, arr);
            arr.add(Integer.valueOf(root.val));
            inOrderTraversal(root.right, arr);
        }
    }
}

/**
 * This is my personal record of solving Leetcode Problems. 
 * If you have any questions, please discuss them in [Issues](https://github.com/mengxinayan/leetcode/issues).
 * Copyright (C) 2020-2022  mengxinayan
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */