Question

Writing a function in C that represents an integer as a singly linked list, the minus sign for negative integers does not show up. What am I doing wrong? Can you suggest any improvements in the algorithm, and the fastest way to solve it? C noob here.

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

typedef struct {
    char info;
    struct Node *link;
} Node;

Node* add_front (Node* head, char info)
{
    Node* t = malloc(sizeof(struct Node));
    t->info = info;
    t->link = head;
    return t;
}

void display (Node* head)
{
    while(head != NULL) {
        printf("%d   ", head->info);
        head = head->link;
    }
}

Node* number_list (int n)
{
    int digit, minus = (n < 0 ? 1 : 0);
    Node* list = NULL;

    if (minus) n *= -1;

    do {
        digit = n % 10;
        list = add_front(list, (char)digit);
        n = n / 10;
    } while(n > 0);

    if (minus) add_front(list, '-');

    return list;
}

int main()
{
        int n = -1024;
        Node* l = number_list(n);
        display(l);
        return 0;
}

Answer 1

if(minus) {
   add_front(list, '-');
}

Since add_front returns the new head of the list, you have to do

if(minus) {
  list =  add_front(list, '-');
}

The next problem is in your display function,

 printf("%d   ", head->info);

Will print the char as a number, printing the char '-' as a number will not print a minus, but the value a - has, 43 in ascii.

Either change the display function to do

while(head != NULL) {
   if(head->info == '-') {
       putchar('-'); 
   }else {
     printf("%d   ", head->info);
   }
    head = head->link;
}

Or store the actual character digit in your list, and print the elements with printf("%c", head->info);. That is, instead of e.g. storing the number 7 in the list, you store the character digit '7'
Which you can do with

list = add_front(list, (char)('0'+digit));