linkedlistcycle2.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if (head == NULL)
        {
            return NULL;
        }
        auto fast = head;
        auto slow = head;
        bool flag = false;
        while(fast != NULL && fast->next != NULL)
        {
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast)
            {
                flag = true;
                break;
            }
        }
        if (flag == false)
        {
            return NULL;
        }
        fast = head;
        while (fast != slow)
        {
            fast = fast->next;
            slow = slow->next;
        }
        return slow;
    }
};