As we are using array datatype for children, its length is always constraint as the size is defined during initialization.

So the below code to check the presence of children for a node will always return 26 irrespective whether there is actual value or null)

return current.children.length == 0; // Return true if no children exist

We Should use below code instead

 for (int i = 0; i < 26; i++) {
            if (node.children[i] != null) {
                return true;
            }
        }
        return false;