判断链表有没有环，用Floyd Cycle Detection算法，用两个快慢指针。

class Solution {public:    bool hasCycle(ListNode *head) {        if (!head) return false;        ListNode *slow, *fast;        slow=fast=head;        do{            if (fast==NULL || fast->next==NULL) return false;            slow = slow->next;            fast = fast->next->next;        }while(slow!=fast);        return true;    }};

 

142是141的进阶，需要额外判断环的起点。

详见 https://leetcode.com/problems/linked-list-cycle-ii/solution/ 中的推导，不过里面应该是 F=(n-1)(a+b)+b，因此从head和相遇点开始同时走，一定会在循环起点相遇。

class Solution {public:    ListNode *detectCycle(ListNode *head) {        if (!head) return NULL;        ListNode *slow, *fast;        slow = fast = head;        do{            if (!fast || !fast->next) return NULL;            slow = slow->next;            fast = fast->next->next;        }while(slow!=fast);                ListNode *p=head, *q=slow;        while(p!=q){            p = p->next;            q = q->next;        };        return p;        }};