Validate Binary Search Tree

public class Solution {
    public boolean isValidBST(TreeNode root) {
       return is(root,Integer.MAX_VALUE,Integer.MIN_VALUE);
    }
    public boolean is(TreeNode root,int max,int min){
        if(root==null)
            return true;
        if(root.val<max && root.val>min){
            return is(root.left,root.val,min)&&is(root.right,max,root.val);
        }
        else return false;
    }
}