【LeetCode142】环形链表 II

/**
 * 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 || head->next == NULL){
            return NULL;
        }
        ListNode* slow = head;
        ListNode* fast = head;
        bool flag = false;
        while(fast != NULL && fast->next != NULL){
            slow = slow->next;
            fast = fast->next->next;
            if(fast == slow){
                flag = true;
                break;
            }
        }
        ListNode* fast2 = head;
        while(fast2 != slow && flag == true){
            fast2 = fast2->next;
            slow = slow->next;
        }
        if (flag == false){
            return NULL;
        }else{
            return fast2;
        }
    }
};