full example solution for the second set of practice problems in "intro to recursion with C++" here: https://wheelercj.github.io/notes/pages/20220502232349.html

main.cpp

#include <iostream>
using namespace std;

struct Node {
	int value = 0;
	Node* next = NULL;
	Node(int new_value) {
		value = new_value;
	}
};

void append(int new_value, Node** next_ptr);
void print_list(Node* node_ptr);
int find_sum(Node* node_ptr);
int find_length(Node* node_ptr);
int find_value(int target, Node* node_ptr, int i);
void insert(int new_value, int index, Node** next_ptr);
void delete_list(Node* head);

int main() {
	Node* list_head = NULL;
	append(11, &list_head);
	append(12, &list_head);
	append(13, &list_head);
	append(14, &list_head);

	print_list(list_head);
	int sum = find_sum(list_head);
	cout << "\nsum = " << sum;
	int length = find_length(list_head);
	cout << "\nlength = " << length;
	int index = find_value(12, list_head, 0);
	cout << "\nindex of 12 = " << index;

	cout << endl;
	insert(8, 3, &list_head);
	print_list(list_head);

	delete_list(list_head);
	cout << endl;
	return 0;
}

void append(int new_value, Node** next_ptr) {
	if (*next_ptr == NULL)
		*next_ptr = new Node(new_value);
	else
		append(new_value, &(*next_ptr)->next);
}

void print_list(Node* node_ptr) {
	if (node_ptr == NULL)
		return;
	cout << node_ptr->value << " ";
	print_list(node_ptr->next);
}

int find_sum(Node* node_ptr) {
	if (node_ptr == NULL)
		return 0;
	return node_ptr->value + find_sum(node_ptr->next);
}

int find_length(Node* node_ptr) {
	if (node_ptr == NULL)
		return 0;
	return 1 + find_length(node_ptr->next);
}

int find_value(int target, Node* node_ptr, int i) {
	if (node_ptr == NULL)
		return -1;
	if (target == node_ptr->value)
		return i;
	return find_value(target, node_ptr->next, i + 1);
}

void insert(int new_value, int index, Node** next_ptr) {
	if (index == 0) {
		Node* new_node = new Node(new_value);
		new_node->next = *next_ptr;
		*next_ptr = new_node;
	}
	else if (*next_ptr == NULL)
		cout << "Error! Invalid index." << endl;
	else
		insert(new_value, index - 1, &(*next_ptr)->next);
}

void delete_list(Node* head) {
	if (head == NULL)
		return;
	delete_list(head->next);
	delete head;
	head = NULL;
}