The resulting linked list consists of only one element when there should be at least 3

Question

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


struct ListNode
{
    int val;
    struct ListNode *next;
};
 
struct ListNode* addTwoNumbers(struct ListNode* l1, struct ListNode* l2){
    struct ListNode *t1 = l1, *t2 = l2;
    struct ListNode *result = NULL, *t3 = NULL;
    int carry = 0;
    int i = 0;
    while (t1 && t2)
    {       
        struct ListNode *temp = (struct ListNode*)malloc(sizeof(struct ListNode));
        temp->next = NULL;

        int sum = t1->val + t2->val + carry;
        carry = sum / 10;
        temp->val = sum % 10;

        t1 = t1->next;
        t2 = t2->next;
        if (!result)
        {
            result = temp;
            t3 = result;
            continue;
        }
        t3->next = temp;
        t3 = temp;
        
        
    }
    if (!t1)
    {
        t1 = t2;
    }
    while (t1)
    {
        if (!t3)
        {
            t3 = (struct ListNode*)malloc(sizeof(struct ListNode));
            t3->next = NULL;
        }
        
        int sum = t1->val + carry;
        carry = sum / 10;
        t3->val = sum % 10;

        t1 = t1->next;
        t3 = t3->next;
    }
    return result;
    


}

int main()
{
    struct ListNode *l1 = NULL, *l2 = NULL, *t1 = NULL, *t2 = NULL, *result = NULL;
    int l1i[] = {2, 4, 3};
    int l2i[] = {5, 6, 4};
    l1 = (struct ListNode*)malloc(sizeof(struct ListNode));
    l1->val = l1i[0];
    l1->next =  t1;
    t1 = NULL;
    printf("%d\t", l1->val);
    for (int i = 1; i < 3; i++)
    {
        if (!t1)
        {
            t1 = (struct ListNode*)malloc(sizeof(struct ListNode));
            t1->val = l1i[i];
            t1->next = NULL;
        }
        printf("%d\t", t1->val);
    }
    printf("\n");
    l2 = (struct ListNode*)malloc(sizeof(struct ListNode));
    l2->val = l2i[0];
    l2->next = t2;
    t2 = NULL;
    printf("%d\t", l2->val);
    for (int i = 1; i < 3; i++)
    {
        if (!t2)
        {
            t2 = (struct ListNode*)malloc(sizeof(struct ListNode));
            t2->val = l2i[i];
            t2->next = NULL;
        }
        printf("%d\t", t2->val);
    }
    printf("\n");

    result = addTwoNumbers(l1, l2);
    t1 = result;
    while (t1)
    {
        printf("%d\t", t1->val);
        t1 = t1->next;
    }
    printf("\n");
    while (l1)
    {
        t1 = l1->next;
        free(l1);
        l1 = t1;
    }
    while (l2)
    {
        t2 = l2->next;
        free(l2);
        l2 = t2;
    }
    while (result)
    {
        t1 = result->next;
        free(result);
        result = t1;
    }

}

The program is suppose to add the values of 2 linked list and store it in a 3rd linked list. The above program compiles but the output list only contains single element. Am I missing something here.

I think the way I am storing and going to the next element of the linked list is the problem.

[don't cast malloc](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Barmar, Mar 24 '23 at 21:05
Your loops in `main()` are not linking the new nodes to the `next` pointer of the previous node. So your lists only have 1 node. — Barmar, Mar 24 '23 at 21:09

score 0 · Answer 1 · answered Mar 24 '23 at 21:14

You're not creating the original lists correctly. You're never linking the new nodes via the next pointer in the previous node.

l1 = malloc(sizeof(struct ListNode));
l1->val = l1i[0];
l1->next = NULL;

struct ListNode *t1 = l1;
for (int i = 1; i < 3; i++)
{
    struct ListNode *t2 = malloc(sizeof(struct ListNode));
    t2->val = l1i[i];
    t2->next = NULL;
    t1.next = t2;
    t1 = t1.next;
}

Do l2 the same way.

score 0 · Answer 2 · answered Mar 24 '23 at 21:36

I think the problem is the setting of result in the addTwoNumbers function;

if (!result)
    {
        result = temp;
        t3 = result;
        continue;
    }

That section will only run once, that is, when result is NULL.

Try this;

if (!result)
    {
        result = temp;
        t3 = result;
        continue;
    }else{
        result->next = temp;
    }

The resulting linked list consists of only one element when there should be at least 3

2 Answers2