链表中环

思路：

1.将ListNode一个个放入set，并检查是否已经存在set中

2.双指针，快慢指针

判断是否有环比较简单，找环的起点浮复杂一些。

#include <iostream>
#include <set>
using namespace std;

struct ListNode
{
    int val;
    ListNode* next;
    ListNode(int x) : val(x), next(NULL) {}
};

ListNode* findloop(ListNode* head)
{
    set<ListNode*> set;
    while(head)
    {
        if(set.find(head) != set.end())
            return head;
        set.insert(head);
        head = head->next;
    }
    return NULL;
}

ListNode* findloop1(ListNode* head)
{
    ListNode* prehead = head;
    ListNode* fast = prehead;
    ListNode* slow = prehead;
    while(fast)
    {
        slow = slow->next;
        fast = fast->next;
        if(!fast)   //至少要走两个节点
            return NULL;
        fast = fast->next;
        if(slow == fast)
            break;
    }
    cout << fast->val<< endl;
    cout << slow->val<< endl;
    if(slow != fast)
        return NULL;
    while(1)    //一定有环了
    {
        if(slow == prehead)
            return prehead;
        prehead = prehead->next;
        slow = slow->next;
    }
    return NULL;
}

int main()
{
    ListNode l1(7);
    ListNode l2(8);
    ListNode l3(9);
    ListNode a(0);
    ListNode b(1);
    ListNode c(2);
    ListNode d(3);
    ListNode f(4);
    ListNode e(5);
    ListNode g(6);
    l1.next = &l2;
    l2.next = &a;
    l2.next = &a;
    a.next = &b;
    b.next = &c;
    c.next = &d;
    d.next = &f;
    f.next = &e;
    e.next = &a;
    cout << findloop1(&l1)->val << endl;
    return 0;
}