LeetCode_Add Two Numbers

ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(l1 == NULL) return l2;
        if(l2 == NULL) return l1;

        std::vector<int> a,b,c;
        while(l1){
          a.push_back(l1->val);
          l1 = l1->next;
        }
        while(l2){
          b.push_back(l2->val);
          l2 = l2->next;
        }

        int i;
        if(a.size() < b.size()){
          c = a;
          a = b;
          b = c;
        }
       
        for(i =0;i <b.size();i++){
          a[i] = a[i] + b[i];
          if(a[i] >= 10)
          {
             a[i] = a[i] -10;
                if(i+1<a.size())
                  a[i+1] += 1;
                else
                  a.push_back(1);
          }
        }
        for(;i<a.size(); i++)
        {
            if(a[i] >= 10)
            {
                a[i] =a[i] - 10;
                if(i+1 < a.size())
                      a[i+1] += 1;
                else
                    a.push_back(1);
                
            }
            
        }
       ListNode *head,*current,*p;
       head = new ListNode(a[0]);
       current = head;
       for(i = 1; i<a.size(); i++){
       
        p = new ListNode(a[i]);
        current->next = p;
        current = p;
       }

      return head;
    }