Solution.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        if (root==null) {
            return true;
        }
        List<Integer> list=inorder(root,new ArrayList<Integer>());
        if (list.size()<2) {
            return true;
        }
        for (int i = 0; i < list.size()-1; i++) {
            if (list.get(i)>=list.get(i+1)) {
                return false;
            }
        }
        return true;
    }
    public List<Integer> inorder(TreeNode root,List<Integer> list){  
        if (root!=null) {  
            list=inorder(root.left,list);  
            list.add(root.val);  
            list=inorder(root.right,list);  
        }  
        return list;  
    }  
}