Function returns None no matter what

Question

I have this function which is supposed to give the value of a linked list node by its index. I can print the value the line before but when I return the value it somehow turns into None.

class ListNode:
 def __init__(self, val=0, next_node=None):
 self.val = val
 self.next = next_node
def create_list(linkedlist, i, j=1):
 if i == 0:
 return
 l = ListNode(j)
 linkedlist.next = l
 create_list(linkedlist.next, i-1, j+1)
def index(head, idx=0):
 if idx == 0:
 return head.val
 print(idx)
 index(head.next, idx-1)
link = ListNode()
create_list(link, 5)
print(index(link, 4))

Output:

4
3
2
1
None

I even returned an integer in the function but that turned into None as well.

Any help would be much appreciated.

• adir abargil: Where is the second return statement?
• user13406539: I don't understand.What I was trying to say was that instead of "return head.val" I just put "return 0" and it returned None.Thanks for responding.
• adir abargil: See answer ... help

Answer 1

You have to return the recursive call as well:

def index(head, idx=0):
 if idx == 0:
 return head.val
 print(idx)
 return index(head.next, idx - 1)

You're missing the return on the last line.

For the other function

def create_list(linkedlist,i,j=1):
 if i == 0:
 return linkedlist # missing object to return in terminal case
 l = ListNode(j)
 linkedlist.next = l
 return create_list(linkedlist.next,i-1,j+1) # missing return on recursive call

• user13406539: Oh I didn't know that.Thanks for the help.
• user13406539: Well in the "create_list" function since it just references them I didn't notice the problem and that kinda confused me. Thanks.