solution.cpp

#include <vector>
#include <algorithm>
#include <sstream>

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        if (!l1)
        {
            return l2;
        }
        else if (!l2)
        {
            return l1;
        }
        
        ListNode* trav1{l1};
        ListNode* trav2{l2};
        vector<int> l1_vec;
        vector<int> l2_vec;
        std::size_t l1_size{};
        std::size_t l2_size{};
        while (trav1) 
        {
            l1_vec.push_back(trav1->val);
            trav1 = trav1->next;
            ++l1_size;
        }
        
        while (trav2)
        {
            l2_vec.push_back(trav2->val);
            trav2 = trav2->next;
            ++l2_size;
        }
        
        if (l1_size == 1 && l2_size == 1)
        {
            int sum{l1_vec[0] + l2_vec[0]};
            ListNode* node{new ListNode(sum, nullptr)};
            return node;
        }
        
        vector<string> str_nums1;
        for (std::size_t i{}; i < l1_vec.size(); ++i)
        {
            str_nums1.push_back(std::to_string(l1_vec[i]));
        }
        ostringstream v_str;
        std::copy(str_nums1.begin(), str_nums1.end(), ostream_iterator<string>(v_str));
        string str_num1{v_str.str()};
        
        vector<string> str_nums2;
        for (std::size_t i{}; i < l2_vec.size(); ++i)
        {
            str_nums2.push_back(std::to_string(l2_vec[i]));
        }
        ostringstream v_str2;
        std::copy(str_nums2.begin(), str_nums2.end(), ostream_iterator<string>(v_str2));
        string str_num2{v_str2.str()};
        
        int sum{std::stoi(str_num1) + std::stoi(str_num2)};
        vector<ListNode*> sum_vec;
        while (sum >= 10)
        {
            ListNode* node{new ListNode(sum % 10, nullptr)};
            sum_vec.push_back(node);
            sum /= 10;
        }
        ListNode* node{new ListNode(sum, nullptr)};
        sum_vec.push_back(node);  // last digit
        for (std::size_t i{}; i < sum_vec.size(); ++i)
        {
            sum_vec[i]->next = sum_vec[i + 1];
        }
        sum_vec[sum_vec.size() - 1]->next = nullptr;
        
        return sum_vec[0];
    }
};