/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private List<Integer> list = new ArrayList<Integer>();
    public boolean isValidBST(TreeNode root) {
        inorderTraversal(root);
        int size = list.size();
        if(size==0 || size==1){
            return true;
        }
        for(int i=0;i<size-1;i++){
            if((list.get(i)) >= (list.get(i+1))){
                return false;
            }
        }
        return true;
    }
    
    //中序遍历
    public void inorderTraversal(TreeNode root){
        if(root==null){
            return;
        }else{
            inorderTraversal(root.left);
            list.add(root.val);
            inorderTraversal(root.right);
        }
    }
}