Add Two Numbers II in C++

Suppose we have two non-empty linked lists representing two non-negative integers. Here the most significant digit comes first and each of their nodes contain a single digit. We have to adf the two numbers and return it as a linked list. So if the lists are [7, 2, 4, 3] + [5, 6, 4], then the result will be [7, 8, 0, 7]

To solve this, we will follow these steps −

• Create a node called dummy, and store value 0, create stacks s1 and s2.

• fill s1 by the nodes of l1 and fill s2 by the nodes of s2.

• sum := 0

• while s1 is not empty or s2 is not empty

  • if s1 is empty, then sum := sum + top value of s1, delete from stack s1

  • if s2 is empty, then sum := sum + top value of s2, delete from stack s2

  • value of dummy := sum mod 10

  • new node := create a new node with the value sum/10

  • next of new node := dummy

  • dummy := new node

  • sum := sum / 10

• if dummy value is 0, then return next of dummy, otherwise dummy.

Example (C++)

Let us see the following implementation to get a better understanding −

#include <bits/stdc++.h>
using namespace std;
class ListNode{
   public:
   int val;
   ListNode *next;
   ListNode(int data){
      val = data;
      next = NULL;
   }
};
ListNode *make_list(vector<int> v){
   ListNode *head = new ListNode(v[0]);
   for(int i = 1; i<v.size(); i++){
      ListNode *ptr = head;
      while(ptr->next != NULL){
         ptr = ptr->next;
      }
      ptr->next = new ListNode(v[i]);
   }
   return head;
}
void print_list(ListNode *head){
   ListNode *ptr = head;
   cout << "[";
   while(ptr){
      cout << ptr->val << ", ";
      ptr = ptr->next;
   }
   cout << "]" << endl;
}
class Solution {
   public:
   ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
      ListNode* dummy;
      dummy = new ListNode(0);
      stack <ListNode*> s1, s2;
      while(l1){
         s1.push(l1);
         l1 = l1->next;
      }
      while(l2){
         s2.push(l2);
         l2 = l2->next;
      }
      int sum = 0;
      while(!s1.empty() || !s2.empty()){
         if(!s1.empty()){
            sum += s1.top()->val;
            s1.pop();
         }
         if(!s2.empty()){
            sum += s2.top()->val;
            s2.pop();
         }
         dummy->val = (sum % 10);
         ListNode* newNode = new ListNode(sum / 10);
         newNode->next = dummy;
         dummy = newNode;
         sum /= 10;
      }
      return dummy->val == 0? dummy->next : dummy;
   }
};
main(){
   vector<int> v1 = {7,2,4,3};
   ListNode *h1 = make_list(v1);
   vector<int> v2 = {5,6,4};
   ListNode *h2 = make_list(v2);
   Solution ob;
   print_list(ob.addTwoNumbers(h1, h2));
}

Input

[7,2,4,3]
[5,6,4]

Output

[7, 8, 0, 7, ]