Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Note: Do not modify the linked list.

/** * 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) {        ListNode* slow = head;        ListNode* fast = head;        while(fast && fast->next){            slow = slow->next;            fast = fast->next->next;            if(slow == fast){                ListNode* slow2 = head;                while(slow2 != slow){                    slow = slow->next;                    slow2 = slow2->next;                 }                return slow;            }        }        return nullptr;    }};