142.Linked List II

ListNode *detectCycle(ListNode *head) {
    if (head == NULL || head->next == NULL)
        return NULL;

    ListNode *slow  = head;
    ListNode *fast  = head;
    ListNode *entry = head;

    while (fast->next && fast->next->next) {
        slow = slow->next;
        fast = fast->next->next;
        if (slow == fast) {                      // there is a cycle
            while(slow != entry) {               // found the entry location
                slow  = slow->next;
                entry = entry->next;
            }
            return entry;
        }
    }
    return NULL;                                 // there has no cycle
}

• http://www.cnblogs.com/hiddenfox/p/3408931.html
• https://discuss.leetcode.com/topic/5284/concise-o-n-solution-by-using-c-with-detailed-alogrithm-description