two-lists-sum.cpp

// Time:  O(n)
// Space: O(1)

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    /**
     * @param l1: the first list
     * @param l2: the second list
     * @return: the sum list of l1 and l2
     */
    ListNode *addLists(ListNode *l1, ListNode *l2) {
        ListNode dummy(INT_MIN);
        ListNode *curr = &dummy;

        int carry = 0;
        while (l1 || l2 || carry) {
            int a = l1? l1->val : 0, b = l2? l2->val : 0;
            int val = carry + a + b;
            curr->next = new ListNode(val % 10);
            carry = val / 10;

            l1 = l1? l1->next : nullptr;
            l2 = l2? l2->next : nullptr;
            curr = curr->next;
        }
        return dummy.next;
    }
};