/**

 * Definition for binary tree

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode(int x) { val = x; }

 * }

 */

public class Solution {

    private int a=-(1<<31);

    public boolean isValidBST(TreeNode root) {

      return middle(root);

    }

    public boolean middle(TreeNode root)

    {

        if(root==null) return true;

        if(!middle(root.left)) return false;

        if(root.val<=a)

        {

            return false;

        }

        a=root.val;

        return middle(root.right);

    }

}

/**

 * Definition for binary tree

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode(int x) { val = x; }

 * }

 */

public class Solution {

    public boolean isValidBST(TreeNode root) {

        int min=-(1<<31);

        int max=1<<31-1;

       return isValid(root,min,max);

    }

    public boolean isValid(TreeNode root,int min,int max)

    {

        if(root==null) return true;

        if(root.val>min&&root.val<max)

        {

            return isValid(root.left,min,root.val)&&isValid(root.right,root.val,max);

        }

        return false;

    }

}