Linked List Cycle II.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) {
        auto fast = head, slow = head;
        while(fast && fast->next)
        {
            fast = fast->next->next;
            slow = slow->next;
            if(slow == fast)break;
        }
        if(!fast || !fast->next)return nullptr;
        fast = head;
        while(slow != fast)
        {
            fast = fast->next;
            slow = slow->next;
        }
        return slow;
    }
};