https://leetcode.com/problems/intersection-of-two-linked-lists/
将其中一条链表首尾相接，然后对另外一条链表进行环路检测。如果两个链表相交，那么另外一条链表就是带环链表，否则无环。问题转化为linked list cycle II了。
记得最后将链表复原。
C++
class Solution { public:ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {if (!headA || !headB) return NULL;ListNode *tailB = headB;while (tailB->next) tailB = tailB->next;tailB->next = headB;ListNode *ret = detect_cycle(headA);tailB->next = NULL;return ret;}ListNode *detect_cycle(ListNode *head) {bool is_cycle = false;ListNode *slow = head, *fast = head;while (fast && fast->next) {slow = slow->next;fast = fast->next->next;if (slow == fast) { is_cycle = true; break; }}if (!is_cycle) return NULL;ListNode *start = head;while (start != slow) {slow = slow->next;start = start->next;}return start;} };