0445.cpp

class Solution {
public:
  ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
    stack<int> s1, s2;

    while (l1 != nullptr) {
      s1.push(l1->val);
      l1 = l1->next;
    }

    while (l2 != nullptr) {
      s2.push(l2->val);
      l2 = l2->next;
    }

    int sum = 0, c = 0;
    ListNode *res = new ListNode();
    while (!s1.empty() || !s2.empty()) {
      if (!s1.empty()) {
        sum += s1.top();
        s1.pop();
      }
      if (!s2.empty()) {
        sum += s2.top();
        s2.pop();
      }

      res->val = sum % 10;
      c = sum / 10;
      ListNode *newNode = new ListNode(c);
      newNode->next = res;
      res = newNode;
      sum = c;
    }

    return c == 0 ? res->next : res;
  }
};