You often want to know the number of items in a linked list. In the code below (which is copied from the chapter), add a method length() to SingleLinkedList which returns the number of items in the chain. Make sure that it is O(1).

class SingleNode:
    def __init__( self, value, nextnode ):
        self.value = value
        self.nextnode = nextnode
        
class SingleLinkedList:
    def __init__( self ):
        self.head = None
    def add( self, value ):
        node = SingleNode( value, self.head )
        self.head = node
    def remove( self ):
        if self.head != None:
            self.head = self.head.nextnode