[FIXED] using python to complete a LeetCode question #2

Issue

The following is my code which tries to add the two numbers, which are also stored in a linked list in reverse order, and returns the sum as a linked list.

But when I try to run this code in LeetCode, it states this exceeds the time. I assume that it may get stuck in the while loop?

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        result = ListNode()
        carry = 0 
    
        while l1 != None or l2 != None or carry:
            if l1 == None:
                v1 = 0
            else:
                v1 = l1.val
                
            if l2 == None:
                v2 = 0
            else:
                v2 = l2.val
        
            total = v1 + v2 + carry 
            carry = total // 10
            total = total % 10
        
            result.next = ListNode(total)
            
            if l1 != None:
                l1.next
                
            if l2 != None:
                l2.next
        
        return result

Solution

There were few things only which I have changed in the code you wrote.

        result = ListNode(0)
        result_tail = result #### added 
        carry = 0 
    
        while l1 or l2 or carry:
            if l1 == None:
                v1 = 0
            else:
                v1 = l1.val
                
            if l2 == None:
                v2 = 0
            else:
                v2 = l2.val
        
            total = v1 + v2 + carry 
            carry = total // 10
            total = total % 10
        
            result_tail.next = ListNode(total)  ### edited
            result_tail = result_tail.next   ### added
            
            if l1 != None:
                l1 = l1.next ### edited
            else:
                l1 = None   ### added
                
            if l2 != None:
                l2 = l2.next    ### edited
            else:
                l2 = None    ### added
                
        return result.next   ### edited

The major thing I wish you to know is that when you wrote l1.next then you are just giving a command which needs a variable to store the value thus the changes go l1 = l1.next. Similarly, when you wrote the same statement you need a counter else statement where you need to address the condition when if condition fails. Since that condition was not present, the while loop runs infinitely.

The final part is where I added a result_tail. By giving a try and error, I saw that if we are not adding it then the values of the result get updated instead of getting appended.

Lastly, It is not a dumb question. Just a friendly suggestion, if you are super new, instead of writing the medium-level code start with easy codes of all concepts. It will help you to know more about specific deals in python like the dictionary, default dictionary, list comprehension, list traversing, tuples, sets, enumerators and all. You may have done competitive coding earlier but to be honest, a new innings always starts on 0.

Answered By - Shikhar Gupta