public boolean isValidBST(TreeNode root) {

       return getValue(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
       }

    public boolean getValue(TreeNode node, int low, int high){
       if (node == null || node.val==Integer.MAX_VALUE|| node.val==Integer.MIN_VALUE) {
           return true;
       }
       if(node.val>low && node.val<high) {
           return getValue(node.left, low, node.val) && getValue(node.right, node.val, high);
       }

       return false;
    }