Splitting a linked list into odd and even sublists.

main.c

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

typedef struct list_node_t {
    int datum;
    struct list_node_t* next;
} list_node_t;

typedef struct list_t {
    list_node_t* head;
    list_node_t* tail;
    size_t size;
} list_t;

bool list_t_init(list_t* list) {
    if (list == NULL) {
        return false;
    }

    list->head = NULL;
    list->tail = NULL;
    list->size = 0;
    return true;
}

bool list_t_append(list_t* list, int datum) {
    if (list == NULL) {
        return false;
    }

    list_node_t* new_node = malloc(sizeof *new_node);

    if (new_node == NULL) {
        return false;
    }

    new_node->datum = datum;
    new_node->next  = NULL;

    if (list->head == NULL) {
        list->head = new_node;
        list->tail = new_node;
    } else {
        list->tail->next = new_node;
        list->tail = new_node;
    }

    list->size++;
    return true;
}

bool list_t_split_odd_even(const list_t* list,
                                 list_t* odd, 
                                 list_t* even) {

    if (list == NULL || odd == NULL || even == NULL) {
        return false;
    }

    if (odd->size != 0 || even->size != 0) {
        return false;
    }

    list_node_t* current = list->head;
    size_t counter = 1;

    while (current != NULL) {
        if (counter % 2 == 1) {
            list_t_append(odd, current->datum);
        } else {
            list_t_append(even, current->datum);
        }

        ++counter;
        current = current->next;
    }

    return true;
}

void list_t_print(list_t* list) {
    if (list == NULL) {
        printf("NULL");
        return;
    }

    bool first = true;

    for (list_node_t* node = list->head; node != NULL; node = node->next) {
        if (first) {
            first = false;
        }
        else {
            printf(" ");
        }

        printf("%d", node->datum);
    }
}

int main()
{
    list_t list;
    list_t_init(&list);

    for (int datum = 1; datum < 10; ++datum) {
        list_t_append(&list, datum);
    }

    puts("Source list:");
    list_t_print(&list);
    puts("");

    list_t odd_list;
    list_t even_list;

    list_t_init(&odd_list);
    list_t_init(&even_list);

    list_t_split_odd_even(&list, &odd_list, &even_list);

    puts("Odd list:");
    list_t_print(&odd_list);
    puts("");

    puts("Even list:");
    list_t_print(&even_list);

    return EXIT_SUCCESS;
}