142.环形链表II

1 用容器set

2 用哈希表

3 用双指针法

class Solution {
public:
    ListNode* detectCycle(ListNode* head) {
        set<ListNode*> exclude;
        ListNode* p,*res;
        p = head;
        while (p != nullptr)
        {
            pair<set<ListNode*>::iterator,bool> wea = exclude.emplace(p);
            if (wea.second == false)
            {
                res = *(wea.first);
                return res;
            }
            p = p->next;
        }
        return nullptr;
    };
    ListNode* detectCycle1(ListNode* head) {
        map<ListNode*,size_t> exclude;
        ListNode* p, * res;
        p = head;
        while (p)
        {
            ++exclude[p];
            if (exclude[p] == 2)
            {
                return p;
            }
            p = p->next;
        }
        return nullptr;
    };
    ListNode* detectCycle(ListNode* head) {
        ListNode* slow = head, * fast = head;
        while (fast != nullptr) {
            slow = slow->next;
            if (fast->next == nullptr) {
                return nullptr;
            }
            fast = fast->next->next;
            if (fast == slow) {
                ListNode* ptr = head;
                while (ptr != slow) {
                    ptr = ptr->next;
                    slow = slow->next;
                }
                return ptr;
            }
        }
        return nullptr;
    }
};