recursion questions in c

1#include <stdio.h>
2
3unsigned long long int factorial(unsigned int i) {
4
5   if(i <= 1) {
6      return 1;
7   }
8   return i * factorial(i - 1);
9}
10
11int  main() {
12   int i = 12;
13   printf("Factorial of %d is %d\n", i, factorial(i));
14   return 0;
15}

1#include <stdio.h>
2int sum(int n);
3
4int main() {
5    int number, result;
6
7    printf("Enter a positive integer: ");
8    scanf("%d", &number);
9
10    result = sum(number);
11
12    printf("sum = %d", result);
13    return 0;
14}
15
16int sum(int n) {
17    if (n != 0)
18        // sum() function calls itself
19        return n + sum(n-1); 
20    else
21        return n;
22}
23

1#include <stdio.h>
2
3int fibonacci(int i) {
4
5   if(i == 0) {
6      return 0;
7   }
8	
9   if(i == 1) {
10      return 1;
11   }
12   return fibonacci(i-1) + fibonacci(i-2);
13}
14
15int  main() {
16
17   int i;
18	
19   for (i = 0; i < 10; i++) {
20      printf("%d\t\n", fibonacci(i));
21   }
22	
23   return 0;
24}