Reverse a linked-list in-place. https://www.interviewcake.com/question/c/reverse-linked-list

Makefile

test: reverse 
	./reverse

reverse: reverse.c
	gcc -Wall -Werror -o reverse reverse.c

clean:
	rm -f reverse

.PHONY: test reverse

reverse.c

/*
https://www.interviewcake.com/question/python/reverse-linked-list

reverse a linked-list in-place.
*/

#include <stdlib.h>
#include <assert.h>

struct Node_ {
    void *data;
    struct Node_ *next;
};
typedef struct Node_ Node;

Node* mknode(void *data) {
    Node* n = malloc(sizeof(Node));
    n->data = data;
    n->next = NULL;
    return n;
}

Node* rev_recur(Node *n, Node *parent) {
    if (n->next == NULL) {
        n->next = parent;
        return n;
    } else {
        Node *new_head = rev_recur(n->next, n);
        n->next = parent;
        return new_head;
    }
}

Node* reverse(Node *list) {
    return rev_recur(list, NULL);
}

void test() {
    Node *head = mknode("a");
    head->next = mknode("b");
    head->next->next = mknode("c");

    head = reverse(head);

    assert(head->data == "c");
    assert(head->next->data == "b");
    assert(head->next->next->data == "a");
}

int main(int argc, char **argv) {
    test();
    return EXIT_SUCCESS;
}