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 {
    /**
     * 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
        // 二叉搜索树满足中序遍历
        List<Integer> list = new ArrayList<>();
        sort(root, list);

        int a = list.get(0);
        for (int i = 1; i < list.size(); i++) {
            if (a >= list.get(i)) {
                return false;
            }
            a = list.get(i);
        }

        return true;
    }

    public void sort (TreeNode root, List<Integer> list) {
        // write code here
        // 二叉搜索树满足中序遍历
        if (root == null) {
            return;
        }
        sort(root.left, list);
        list.add(root.val);
        sort( root.right, list);
    }
}