142. Linked List Cycle II(两个指针)

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/**
 * 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;
        }
        ListNode *fast = head;
        ListNode *slow = head;
        ListNode *meet = NULL, *p = head;
        while(fast && fast->next){
            slow = slow->next;
            fast = fast->next->next;
            if(slow == fast ){
                slow = head;
                while(fast != slow){
                    slow = slow->next;
                    fast = fast->next;
                }
                return slow;
            }
        }
        return NULL;
    }
};