2-6.cpp

#include <iostream>
//#include <string>
using namespace std;
class Node
{
public:
    char value;
    Node *next;
    Node(int v) : value(v) {}
};
char find_loop(Node *head)
{
    Node *slow = head;
    Node *fast = head;
    //------------------------------
    slow = slow->next;
    if (fast->next != NULL) {
        fast = fast->next->next;
    }
    //------------------------------
    //cout << (slow == fast) << endl; 
    while (fast->next != NULL && slow != NULL && fast != slow) {
        //cout << "y" << endl;
        slow = slow->next;
        fast = fast->next->next;
    }
    if (slow != fast) {
        return '|';
    }
    //cout << slow->value << endl;
    //restart
    slow = head;
    while (slow != fast) {
        slow = slow->next;
        fast = fast->next;
    }
    return slow->value;


}

int main()
{
    /* 'd' is the start of loop */
    Node *a = new Node('a');
    Node *b = new Node('b');
    Node *c = new Node('c');
    Node *d = new Node('d');
    Node *e = new Node('e');
    Node *f = new Node('f');
    Node *g = new Node('g');
    Node *h = new Node('h');
    Node *i = new Node('i');
    Node *j = new Node('j');
    Node *k = new Node('k');
    a->next = b;
    b->next = c;
    c->next = d;
    d->next = e;
    e->next = f;
    f->next = g;
    g->next = h;
    h->next = i;
    i->next = j;
    j->next = k;
    k->next = d;
    cout << find_loop(a) << endl;
    return 0;
}