Add two numbers represented by linked lists?

Here we will see how to add two numbers stored into separate linked list. In the linked list, each digit of the numbers is stored. If the number is 512, then it will be stored like below −

512 = (5)-->(1)-->(2)-->NULL

We are providing two lists of this type, our task is to add them and get the result after calculating the sum. Here we are using the C++ STL linked list. Let us see the algorithm to bet better idea.

Algorithm

addListNumbers(l1, l2)

Begin
   Adjust the l1 and l2 lengths by adding leading 0s with the smaller one
   carry := 0
   res := an empty list
   for each node n from l1, scan from last to first, do
      item := (l1.item + l2.item + carry) mod 10
      insert item at the beginning of res
      carry := (l1.item + l2.item + carry) / 10
   done
   if carry is not 0, then
      add carry at the beginning of res
   end if
   return res
End

Example

#include<iostream>
#include<list>
using namespace std;
list addListNumbers(list<int> l1, list<int> l2){
   //add leading 0s to the shortest number to make them equal length
   if(l1.size() > l2.size()){
      for(int i = l2.size(); i != l1.size(); i++){
         l2.push_front(0);
      }
   }else if(l1.size() < l2.size()){
      for(int i = l1.size(); i != l2.size(); i++){
         l1.push_front(0);
      }
   }
   list<int>::reverse_iterator it1 = l1.rbegin();
   list<int>::reverse_iterator it2 = l2.rbegin();
   list<int> result;
   int carry = 0;
   while(it1 != l1.rend()){
      result.push_front((*it1 + *it2 + carry) % 10);
      carry = (*it1 + *it2 + carry) / 10;
      it1++; it2++;
   }
   if(carry != 0){
      result.push_front(carry);
   }
   return result;
}
list<int> numToList(int n){
   list<int> numList;
   while(n != 0){
      numList.push_front(n % 10);
      n /= 10;
   }
   return numList;
}
void displayListNum(list<int> numList){
   for(list<int>::iterator it = numList.begin(); it != numList.end();
   it++){
      cout<<*it;
   }
   cout << endl;
}
int main() {
   int n1 = 512;
   int n2 = 14578;
   list<int> n1_list = numToList(n1);
   list<int> n2_list = numToList(n2);
   list<int> res = addListNumbers(n1_list, n2_list);
   cout << "First number: "; displayListNum(n1_list);
   cout << "Second number: "; displayListNum(n2_list);
   cout << "Result: "; displayListNum(res);
}

Output

First number: 512
Second number: 14578
Result: 15090