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
        //中序遍历
        ArrayList<Integer> list = new ArrayList<Integer>();
        addNode(root, list);
        for(int i = 0; i < list.size()-1;i++){
            if(list.get(i+1) <= list.get(i)){
                return false;
            }
        }
        return true;
    }
    public void addNode(TreeNode root, ArrayList<Integer> list){
        if(root.left != null){
            addNode(root.left, list);
        }
        list.add(root.val);
        if(root.right != null){
            addNode(root.right, list);
        }

    }
}