142环形链表

最简单的思路：哈希。进阶那个快慢指针确实想不到。

//哈希，空间为O(n)
class Solution {
public:ListNode *detectCycle(ListNode *head) {unordered_set<ListNode*> adds ; if(head == nullptr)return NULL;ListNode* cur = head;while(cur->next != nullptr){if(adds.count(cur))return cur;adds.insert(cur);cur = cur->next;}       return NULL;}
};
//使用快慢指针 空间为O(1)
class Solution {
public:ListNode *detectCycle(ListNode *head) {ListNode* fast = head;ListNode* slow = head;while(fast != nullptr && fast->next != nullptr){fast = fast->next->next;slow = slow->next;if( fast == slow ){ListNode* index1 = head;ListNode* index2 = fast;while(index1 != index2){index1 = index1->next;index2 = index2->next;}return index1;}}return NULL;}
};