Given a reference ( pointer to pointer ) to the head of a list and an int , inserts a new node on the front of the list .
static Node push ( Node head_ref , char new_data ) { Node new_node = new Node ( ) ; new_node . data = new_data ; new_node . next = head_ref ; new_node . prev = null ; if ( head_ref != null ) head_ref . prev = new_node ; head_ref = new_node ; return head_ref ; }