力扣经典题：环形链表的检测与返回

1.值得背的题

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode *detectCycle(struct ListNode *head) {
    struct ListNode*fast=head;
    struct ListNode*slow=head;
    while(fast!=NULL&&fast->next!=NULL)
    {
        slow=slow->next;
        fast=fast->next->next;
        if(slow==fast)
        {
            struct ListNode*good=fast;
            struct ListNode*bad=head;
            while(good!=bad)
            {
                good=good->next;
                bad=bad->next;
            }
            return bad;
        }
    }
    return NULL;
}