Add Two Numbers in Python

Suppose we have given two non-empty linked lists. These two lists are representing two non-negative integer numbers. The digits are stored in reverse order. Each of their nodes contains only one digit. Add the two numbers and return the result as a linked list. We are taking the assumption that the two numbers do not contain any leading zeros, except the number 0 itself. So if the numbers are 120 + 230, then the linked lists will be [0 → 2 → 1] + [0 → 3 → 2] = [0 → 5 → 3] = 350.

To solve this, we will follow these steps

• Take two lists l1 and l2. Initialize head and temp as null
• c := 0
• while l1 and l2 both are non-empty lists
  • if l1 is non-empty, then set a := 0, otherwise set a := l1.val
  • if l2 is non-empty, then set b := 0, otherwise set b := l2.val
  • n := a + b + c
  • if n > 9, then c := 1 otherwise 0
  • node := create a new node with value n mod 10
  • if head is null

    • head := node and temp := node

  • otherwise
    • head.next := node, and head := node
  • l1 := next node of l1, if l1 exists
  • l2 := next node of l2, if l2 exists
• if c is non-zero, then
  • node := new node with value 1, next of head := node
• return temp

Example(Python)

Let us see the following implementation to get a better understanding

 Live Demo

class ListNode:
   def __init__(self, data, next = None):
      self.val = data
      self.next = next
def make_list(elements):
   head = ListNode(elements[0])
   for element in elements[1:]:
      ptr = head
      while ptr.next:
         ptr = ptr.next
      ptr.next = ListNode(element)
   return head
def print_list(head):
   ptr = head
   print('[', end = "")
   while ptr:
      print(ptr.val, end = ", ")
      ptr = ptr.next
   print(']')
class Solution:
   def addTwoNumbers(self, l1: ListNode, l2: ListNode) -> ListNode:
      head = None
      temp = None
      c = 0
      while l1 or l2:
         if not l1:
            a= 0
         else:
            a = l1.val
         if not l2:
            b=0
         else:
            b = l2.val
         n = a +b + c
         c = 1 if n>9 else 0
         node = ListNode(n%10)
         if not head:
            head = node
            temp = node
         else:
            head.next = node
            head = node
         l1 = l1.next if l1 else None
         l2 = l2.next if l2 else None
      if c:
         node = ListNode(1)
         head.next = node
      return temp
ob1 = Solution()
l1 = make_list([0,2,1])
l2 = make_list([0,3,2])
print_list(ob1.addTwoNumbers(l1, l2))

Input

[0,2,1]
[0,3,2]

Output

[0,5,3]