445

/**
 * 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) {
        stack<int> num1, num2, ans;
        while(l1){
            num1.push(l1->val);
            l1 = l1->next;
        }
        while(l2){
            num2.push(l2->val);
            l2 = l2->next;
        }

        int carry = 0, sum;
        while(!num1.empty() || !num2.empty()){
            sum = 0;
            if(!num1.empty()){
                sum += num1.top();
                num1.pop();
            }
            if(!num2.empty()){
                sum += num2.top();
                num2.pop();
            }
            sum += carry;
            ans.push(sum%10);
            carry = sum/10;
        }
        while(carry){
            ans.push(carry%10);
            carry /= 10;
        }

        ListNode* head = new ListNode();
        ListNode* curr = head;
        while(!ans.empty()){
            curr->next = new ListNode(ans.top());
            curr = curr->next;
            ans.pop();
        }
        return head->next;
    }
};