solved "Linked List Cycle" I and II on LeetCode (the algorithm of fast runner and slow runner is awesome!! and the solution to the second is even more mind-blowing :) http://oj.leetcode.com/problems/linked-list-cycle/ *and* http://oj.leetcode.com/problems/linked-list-cycle-ii/

linked_list_cycle.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 
// GREAT algorithm! inspired by that discussion forum.
 
class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode *fast = head, 
                 *slow = head;
        do {
            if (fast == NULL || fast->next == NULL) 
                return 0;
            else {
                fast = fast->next->next;
                slow = slow->next;
            }
        } while (fast != slow);
        return 1;
    }
};

// Previous prob's variation
// check my blog

class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        ListNode *fast = head, 
                 *slow = head;
        do {
            if (fast == NULL || fast->next == NULL) 
                return NULL;
            else {
                fast = fast->next->next;
                slow = slow->next;
            }
        } while (fast != slow);
        slow = head;
        while (fast != slow) {
            fast = fast->next;
            slow = slow->next;
        }
        return fast;
    }
};