Wrong output in Leetcode problem "Add two numbers"

Question

I'm trying to understand why I get different output when I run the code on leetcode and in VSC. I solved the problem "Add Two Numbers" in Python with the following code:

from types import NoneType


class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None


class Solution(object):
    def addTwoNumbers(self, l1, l2):
        str_l1=""
        str_l2=""
        if type(l1.val) is int:
          my_l1=[l1.val]
        elif type(l1.val) is NoneType:
          my_l1=[0]
        else:
          my_l1=l1.val

        if type(l2.val) is int:
          my_l2=[l2.val]
        elif type(l2.val) is NoneType:
          my_l2=[0]
        else:
          my_l2=l2.val

        for i in range(len(my_l1)):
          str_l1+=str(my_l1[len(my_l1)-i-1])
        
        for i in range(len(my_l2)):
          str_l2+=str(my_l2[len(my_l2)-i-1])
        
        num_l1=int(str_l1)
        num_l2=int(str_l2)
        somma=num_l1+num_l2
        str_somma=str(somma)
        list_somma=[]
        for i in range(len(str_somma)):
          list_somma.append(int(str_somma[len(str_somma)-i-1]))
        sol=ListNode(list_somma)
        return sol

And this is the example:

my_solution=Solution()
x=my_solution.addTwoNumbers(ListNode([2,4,3]),ListNode([5,6,4]))
print(x.val)

When I run the code in VSC I get the correct output [7,0,8], while when I run it in leetcode with the same input I get the wrong output [[7]]. How is that possible?

Had to look up what leetcode is: I just tried the playground and copied your code. For Python and Python3 the ouput is [7, 0, 8] — Enak
– Enak, Commented Aug 6, 2022 at 9:05
I can't reproduce your problem. Maybe you run different code in VSC. — furas
– furas, Commented Aug 6, 2022 at 10:19
Python has special function to check type - isinstance(l1.val, int) but for None I would use is - like elif l1.val is None: — furas
– furas, Commented Aug 6, 2022 at 10:19
Python allow for negatie index and you can use str_somma[-i-1] instead of str_somma[len(str_somma)-i-1]. But you could simply use reversed(my_l1) and run without range() and len() like for item in reversed(my_l1): str_l1 += str(item) — furas
– furas, Commented Aug 6, 2022 at 10:25

slothrop · Accepted Answer · 2022-08-06 09:46:40Z

The expected solution is a linked list, so a simple print(x.val) on one node doesn't display it very helpfully.

You can make your own helper function to print the linked list, something like:

def print_ll(head):
    res = []
    n = head
    while n:
        res.append(n.val)
        n = n.next
    print(res)

If you use your solution and then do print_ll(x), you get [[7, 0, 8]]. Note, it's not [7, 0, 8] which is what you would expect.

The reason is that the correct solution to the problem is a linked list with 3 nodes, each of which has an integer value. But what your code gives is a single node, whose value is itself a list.

Why does LeetCode show a value ([[7]]) which is different again?

Look at the input you give your code when you run in VSCode:

x=my_solution.addTwoNumbers(ListNode([2,4,3]),ListNode([5,6,4]))

The inputs to the function there are not the same as it will get in LeetCode. You are giving inputs which are single nodes with .val equal to a list. But on LeetCode, the inputs are linked lists with multiple nodes. So the result of this is not an accurate reflection of what happens when you run on LeetCode.

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