Give an algorithm for finding the ith-to-last node in a singly linked list in which the last node is indicated by a null next reference.

Algorithm for finding the ith-to-last node in a singly linked list:

1. Initialize two pointers, current and iThToLast, to point to the head of the linked list.
2. Move the current pointer i positions ahead, where i is the desired distance from the last node to the i-th-to-last node.
3. If the current pointer reaches the end of the linked list (i.e., it becomes null), the desired i-th-to-last node does not exist. Return null or an appropriate error indication.
4. Otherwise, move both the current and iThToLast pointers one step at a time until the current pointer reaches the end of the linked list.
5. When the current pointer reaches the end of the linked list, the iThToLast pointer will be pointing to the desired i-th-to-last node.
6. Return the iThToLast node.

The algorithm works by maintaining a distance of i nodes between the current and iThToLast pointers. As the current pointer moves i positions ahead, the iThToLast pointer also moves one position at a time. When the current pointer reaches the end of the linked list, the iThToLast pointer will be pointing to the i-th-to-last node.

Here is an example implementation of the algorithm in C++:

#include <iostream>

struct Node {
    int data;
    Node* next;
};

Node findIthToLastNode(Node head, int i) {
    Node* current = head;
    Node* iThToLast = head;

    // Move the current pointer i positions ahead
    for (int j = 0; j < i; j++) {
        if (current == nullptr) {
            // i is greater than the length of the linked list
            return nullptr;
        }
        current = current->next;
    }

    // Move both pointers one step at a time until the current pointer reaches the end
    while (current != nullptr) {
        current = current->next;
        iThToLast = iThToLast->next;
    }

    return iThToLast;
}

int main() {
    // Create a sample linked list: 1 -> 2 -> 3 -> 4 -> 5 -> nullptr
    Node* head = new Node{1, new Node{2, new Node{3, new Node{4, new Node{5, nullptr}}}}};

    int i = 2; // Find the 2nd-to-last node

    Node* iThToLastNode = findIthToLastNode(head, i);
    if (iThToLastNode != nullptr) {
        std::cout << "The " << i << "-th to last node is: " << iThToLastNode->data << std::endl;
    } else {
        std::cout << "The " << i << "-th to last node does not exist." << std::endl;
    }

    // Clean up the memory
    Node* current = head;
    while (current != nullptr) {
        Node* next = current->next;
        delete current;
        current = next;
    }

    return 0;
}

This implementation creates a sample linked list and finds the i-th-to-last node using the findIthToLastNode function. The Node struct represents a node in the linked list, with data storing the data value and next pointing to the next node in the list. The function returns a pointer to the i-th-to-last node or nullptr if it does not exist.