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Python Program To Add Two Numbers Represented By Linked Lists- Set 1

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  • Last Updated : 18 May, 2022

Given two numbers represented by two lists, write a function that returns the sum list. The sum list is a list representation of the addition of two input numbers.


List1: 5->6->3 // represents number 563 
List2: 8->4->2 // represents number 842 
Resultant list: 1->4->0->5 // represents number 1405 
Explanation: 563 + 842 = 1405 Input: 
List1: 7->5->9->4->6 // represents number 75946
List2: 8->4 // represents number 84
Resultant list: 7->6->0->3->0// represents number 76030
Explanation: 75946+84=76030

Approach: Traverse both lists and One by one pick nodes of both lists and add the values. If the sum is more than 10 then make carry as 1 and reduce sum. If one list has more elements than the other then consider the remaining values of this list as 0. 

The steps are: 

  1. Traverse the two linked lists from start to end
  2. Add the two digits each from respective linked lists.
  3. If one of the lists has reached the end then take 0 as its digit.
  4. Continue it until both the end of the lists.
  5. If the sum of two digits is greater than 9 then set carry as 1 and the current digit as sum % 10

Below is the implementation of this approach. 


# Python program to add two numbers 
# represented by linked list
# Node class
class Node:
    # Constructor to initialize the 
    # node object
    def __init__(self, data): = data = None
class LinkedList:
    # Function to initialize head
    def __init__(self):
        self.head = None
    # Function to insert a new node at 
    # the beginning
    def push(self, new_data):
        new_node = Node(new_data) = self.head
        self.head = new_node
    # Add contents of two linked lists and 
    # return the head node of resultant list
    def addTwoLists(self, first, second):
        prev = None
        temp = None
        carry = 0
        # While both list exists
        while(first is not None or 
              second is not None):
            # Calculate the value of next digit 
            # in resultant list
            # The next digit is sum of following 
            # things
            # (i) Carry
            # (ii) Next digit of first list (if 
            # there is a next digit)
            # (iii) Next digit of second list (if 
            # there is a next digit)
            fdata = 0 if first is None else
            sdata = 0 if second is None else
            Sum = carry + fdata + sdata
            # Update carry for next calculation
            carry = 1 if Sum >= 10 else 0
            # Update sum if it is greater than 10
            Sum = Sum if Sum < 10 else Sum % 10
            # Create a new node with sum as data
            temp = Node(Sum)
            # if this is the first node then set 
            # it as head of resultant list
            if self.head is None:
                self.head = temp
       = temp
            # Set prev for next insertion
            prev = temp
            # Move first and second pointers to 
            # next nodes
            if first is not None:
                first =
            if second is not None:
                second =
        if carry > 0:
   = Node(carry)
    # Utility function to print the 
    # linked LinkedList
    def printList(self):
        temp = self.head
            temp =
# Driver code
first = LinkedList()
second = LinkedList()
# Create first list
print "First List is ",
# Create second list
print "
Second List is ",
# Add the two lists and see result
res = LinkedList()
res.addTwoLists(first.head, second.head)
print "
Resultant list is ",
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)


First List is 7 5 9 4 6 
Second List is 8 4 
Resultant list is 5 0 0 5 6 

Complexity Analysis: 

  • Time Complexity: O(m + n), where m and n are numbers of nodes in first and second lists respectively. 
    The lists need to be traversed only once.
  • Space Complexity: O(m + n). 
    A temporary linked list is needed to store the output number

Related Article: Add two numbers represented by linked lists | Set 2

Please refer complete article on Add two numbers represented by linked lists | Set 1 for more details!

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