LeetCode 2. Add Two Numbers

My code seems to be little bit more complex than the solution provided. I feel like I could have done it more efficiently, so the thing I’ve realized is that I need more time and practice to make myself a good thinker.

I’ve struggled because of the following reasons:

1. It was my first time solving problem on LeetCode. I didn’t know the class ListNode was already implemented in its own system, so I shouldn’t have to implement the new ListNode class, otherwise it will output error messages.

2. I thought I had to implement all the I/O system by myself, but in Link Node problems, the only thing you need to do is finish the function already provided in the problem and submit the head node of the Link List.

My answer:

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

class Solution:
    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
        ptr_first = ptr_result = ListNode()

        flag = 0
        first = True
        while True: 
            add = int(l1.val) + int(l2.val) + flag
            if first: 
                ptr = ListNode(add)
                first = False

            if add > 9: 
                flag = 1
                ptr = ListNode(add - 10)
            else: 
                flag = 0
                ptr = ListNode(add)

            ptr_result.next = ptr  
            ptr_result = ptr  
            l1 = l1.next
            l2 = l2.next

            if l1 == None and l2 == None and flag == 1: 
                ptr = ListNode(1)
                ptr_result.next = ptr

            if l1 == None or l2 == None: 
                break


        # ptr_l2 가 끝났을 때.
        while l1 != None:
            add = int(l1.val) + flag

            if (add > 9):         
                flag = 1  
                ptr = ListNode(add - 10) 
            else: 
                flag = 0     
                ptr = ListNode(add)        

            ptr_result.next = ptr
            ptr_result = ptr       
            l1 = l1.next     

            if l1 == None and flag == 1: 
                ptr = ListNode(1)
                ptr_result.next = ptr


        while l2 != None: 
            add = int(l2.val) + flag

            if (add > 9): 
                flag = 1
                ptr = ListNode(add - 10)
            else: 
                flag = 0
                ptr = ListNode(add)

            ptr_result.next = ptr
            ptr_result = ptr
            l2 = l2.next     

            if l2 == None and flag == 1: 
                ptr = ListNode(1)
                ptr_result.next = ptr

        return ptr_first.next

Solution:

class Solution:
    def addTwoNumbers(
        self, l1: Optional[ListNode], l2: Optional[ListNode]
    ) -> Optional[ListNode]:
        dummyHead = ListNode(0)
        curr = dummyHead
        carry = 0
        while l1 != None or l2 != None or carry != 0:
            l1Val = l1.val if l1 else 0
            l2Val = l2.val if l2 else 0
            columnSum = l1Val + l2Val + carry
            carry = columnSum // 10
            newNode = ListNode(columnSum % 10)
            curr.next = newNode
            curr = newNode
            l1 = l1.next if l1 else None
            l2 = l2.next if l2 else None
        return dummyHead.next

From the next post, I will be posting about not only the minor things or system problems like I did today but also all the logic I’ve developed while I was solving the problem.