/** * 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* fast = head;        ListNode* slow = head;        while (step(fast, 2) && step(slow, 1)) {            if (fast == slow) break;        }        if (fast == NULL) {            return NULL;        }        ListNode* fake_end = slow;                ListNode* h2 = fake_end->next;        ListNode* h1 = head;                ListNode* cur1 = h1;        ListNode* cur2 = h2;                int len1 = 0, len2 = 0;                while (cur1 != fake_end) len1++, cur1 = cur1->next;        while (cur2 != fake_end) len2++, cur2 = cur2->next;                if (len1 == len2) {            len1 = len2 = 0;        } else if (len1 > len2) {            len1 = len1 - len2;            len2 = 0;        } else {            len1 = 0;            len2 = len2 - len1;        }                step(h1, len1);        step(h2, len2);                while (h2 != h1 && step(h1, 1) && step(h2, 1));                return h1;    }        bool step(ListNode* &cur, int n) {        while (cur != NULL && n > 0) {            n--;            cur = cur->next;        }        return n == 0;    }};