https://leetcode.com/problems/add-two-numbers/

add_rev_llist.cpp

struct ListNode {
    int val;
    struct ListNode* next;
    ListNode(int val) : val(val), next(0) {}
};

struct ListNode* newNode(int data) {
    return new struct ListNode(data);
}

struct ListNode* Sum(struct ListNode* l1, struct ListNode* l2, int carry) {
    // add the numbers with the carry (initially 0)
    int sum = carry + l1->val + l2->val;
    // the digit to carry over to the next sum
    int digitToCarry = sum/10;
    // insert the last digit
    struct ListNode* result = newNode(sum % 10);
    if (!l1->next || !l2->next) {
        // check if we are at the end of the list, if so,
        // check if there is another digit to insert and insert it
        if (!l1->next && !l2->next) {
            // a check for sum>=10 is done so that we don't append a trailing 0
            if (sum >= 10) result->next = newNode(digitToCarry);
            return result;
        }
        // if the second list is longer, append zeros to the first
        if (!l1->next) l1->next = newNode(0);
        // if the first list is longer, append zeros to the second
        if (!l2->next) l2->next = newNode(0);
    }
    // compute the rest of the sum and append to result
    result->next = Sum(l1->next, l2->next, digitToCarry);
    return result;
}

struct ListNode* Sum(struct ListNode* l1, struct ListNode* l2) {
    return Sum(l1,l2,0);
}

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        return Sum(l1, l2);
    }
};