这如何打印反向链接列表

Question

How does this piece of code print a linked list reversed? I'm pretty curious how the new list comes to this way.

class Empty:
    def __init__ (self):
        self.IsEmpty = True
Empty = Empty()

class Node:
    def __init__ (self,value,tail):
        self.IsEmpty = False
        self.value = value
        self.tail = tail

l = Node(1,Node(2,Node(3,Node(4,Empty))))   


c = l
nl = Empty

while not l.IsEmpty:
    nl = Node(l.value,nl)
    l = l.tail

while not nl.IsEmpty:
    print(nl.value)
    nl = nl.tail

Answer 1

It doesn't print reversed, nl is built reversed:

while not l.IsEmpty:
    nl = Node(l.value,nl)
    l = l.tail

each new nl is the tail of the next one. So 1 is the tail for 2, and so on.

Answer 2

Think about what is happening in the first while loop. It builds a new list in reverse. We start with Empty . Then each iteration:

new_list = Node(l.current_value, Node(l.previous_value))

Where previous_value is the value encountered first while iterating through the original list. So, this could be expanded to:

Node(l.last_value, Node(l.second_to_last, Node(...Node(l.first_value, Empty)...)))

Answer 3

It builds a forwards list

l = Node(1,Node(2,Node(3,Node(4,Empty))))   

Then builds a backwards list from that forwards list

nl = Empty

while not l.IsEmpty:
    nl = Node(l.value,nl)
    l = l.tail

Then prints that list out in order, which is the reverse of the first list.

This is a pretty convoluted way of doing things. Better would be something recursive, like:

def printer(curr):
    if curr.isEmpty:
        return
    printer(curr.tail)
    print(curr.value)