2.c

#include <stdio.h>
#include <stdlib.h>

struct ListNode {
    int val;
    struct ListNode *next;
};

struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2) {
    if (l1 == NULL) return l2;
    if (l2 == NULL) return l1;

    struct ListNode *p1 = l1, *p2 = l2;
    struct ListNode *ans = (struct ListNode *)calloc(1, sizeof(struct ListNode));
    struct ListNode *p = ans, *last = NULL, *t = NULL;

    int sum = 0;
    while (1) {
        if (p1 && p2) {
            sum += p1->val + p2->val;
            p1 = p1->next;
            p2 = p2->next;
        }
        else if (p1 && (p2 == NULL)) {
            sum += p1->val;
            p1 = p1->next;
        }
        else if (p2 && (p1 == NULL)) {
            sum += p2->val;
            p2 = p2->next;
        }
        else {
            break;
        }

        p->val = sum % 10;
        sum /= 10;

        /* for next node or the final node */
        p->next = t = (struct ListNode *)calloc(1, sizeof(struct ListNode));
        last = p;
        p = p->next;
    }

    if (sum != 0) {
        t->val = sum;
    }
    else {
        /* sum is 0, we don't need a redundant 0, delete it */
        free(t);
        last->next = NULL;
    }

    return ans;
}

int main() {
    struct ListNode *l1 = (struct ListNode *)calloc(3, sizeof(struct ListNode));
    struct ListNode *p1 = l1;
    p1->val = 2;
    p1->next = p1 + 1;
    p1++;

    p1->val = 4;
    p1->next = p1 + 1;
    p1++;

    p1->val = 5;
    p1->next = NULL;

    struct ListNode *l2 = (struct ListNode *)calloc(5, sizeof(struct ListNode));
    struct ListNode *p2 = l2;
    p2->val = 5;
    p2->next = p2 + 1;
    p2++;

    p2->val = 6;
    p2->next = p2 + 1;
    p2++;

    p2->val = 4;
    p2->next = p2 + 1;
    p2++;

    p2->val = 9;
    p2->next = p2 + 1;
    p2++;

    p2->val = 9;
    p2->next = NULL;

    struct ListNode *p = addTwoNumbers(l1, l2);
    
    /* 542 + 99465 = 100007 */
    while (p != NULL) {
        printf("%d ", p->val);
        p = p->next;
    }
    printf("\n");

    return 0;
}