判断单链表是否有环路可以参考：单链表判断环路及环路的入口

typedef int DataType;//节点中数据类型

typedef struct ListNode//节点数据结构
{
    DataType data;
    struct ListNode *next;
} ListNode;

typedef struct CheckNoode/
{
    int flag;
    ListNode *p1;
    ListNode *p2;

} CheckNoode;

1.判断两个链表是否相交，若相交，求交点。（假设不带环）

//判断是否相交
bool CheckIsMeet(ListNode *plist1,ListNode *plist2)
{
    while (plist1->next != NULL)
    {
        plist1 = plist1->next;
    }
    while (plist2->next != NULL)
    {
        plist2 = plist2->next;
    }
    if (plist1 == plist2)
    {
        return true;
    }
    else
    {
        return false;
    }
}

//若相交，求交点
ListNode *AnswerMeetNode(ListNode *plist1,ListNode *plist2){
    int count1 = 0;
    int count2 = 0;
    int count = 0;
    ListNode *cur1 = plist1;
    ListNode *cur2 = plist2;
    while (cur1 != NULL)
    {
        count1++;
        cur1 = cur1->next;
    }
    while (cur2 != NULL)
    {
        count2++;
        cur2 = cur2->next;
    }
    count = count1 - count2;
    if (count > 0)
    {
        while (count--)
        {
            plist1 = plist1->next;
        }
    }
    else
    {
        count = -count;
        while (count--)
        {
            plist2 = plist2->next;
        }
    }
    while (plist1 != plist2)
    {
        plist1 = plist1->next;
        plist2 = plist2->next;
    }
    return plist1;
}

void Test1()
{
    ListNode *plist1 = NULL;
    ListNode *plist2 = NULL;
    bool flag = true;

if (flag)
    {
        printf("相交！\n");
        printf("交点为：%d\n",AnswerMeetNode(plist1,plist2)->data);
    }
    else
    {
        printf("不想交！\n");
    }
}

2.判断两个链表是否相交，若相交，求交点。（假设可能带环）

1).两个都不带环
有可能相交，有可能不相交

2).一个带环一个不带环，必定不相交。

3).两个都带环
不相交
相交：交点不在环上；交点在环上

//判断两个链表是否相交，若相交，求交点.（假设链表可能带环）
CheckNoode CheckIsMeetTop(ListNode *plist1, ListNode *plist2)
{
    ListNode *tmp1 = NULL;
    ListNode *tmp2 = NULL;

    CheckNoode ret = { 0, NULL, NULL };

    tmp1 = IsCycle(plist1);
    tmp2 = IsCycle(plist2);

    if ((tmp1 == NULL) && (tmp2 == NULL))
    {
        //都不带环
        tmp1 = plist1;
        tmp2 = plist2;
        while (tmp1->next != NULL)
        {
            tmp1 = tmp1->next;
        }
        while (tmp2->next != NULL)
        {
            tmp2 = tmp2->next;
        }
        if (tmp1 == tmp2)
        {
            //相交
            ret.plist1 = AnswerMeetNode(plist1, plist2);
            ret.flag = 1;
            return ret;
        }
        else
        {
            //不相交
            ret.flag = 1;
            return ret;
        }
    }
    else if ((tmp1 != NULL) && (tmp2 != NULL))
    {
        //两者都带环
        ListNode *tmp = tmp1->next;
        while ((tmp != tmp2) && (tmp != tmp1))
        {
            tmp = tmp->next;
        }
        if (tmp == tmp2)
        {
            //相交
            tmp1 = GetCycleEnter(plist1,tmp1);
            tmp2 = GetCycleEnter(plist2,tmp2);
            if (tmp1 == tmp2)
            {
                //交点在链上
                int count1 = 0;
                int count2 = 0;
                int count = 0;
                ListNode *cur1 = plist1;
                ListNode *cur2 = plist2;
                while (cur1 != tmp1)
                {
                    count1++;
                    cur1 = cur1->next;
                }
                while (cur2 != tmp1)
                {
                    count2++;
                    cur2 = cur2->next;
                }
                count = count1 - count2;
                if (count > 0)
                {
                    while (count--)
                    {
                        plist1 = plist1->next;
                    }
                }
                else
                {
                    count = -count;
                    while (count--)
                    {
                        plist2 = plist2->next;
                    }
                }
                while (plist1 != plist2)
                {
                    plist1 = plist1->next;
                    plist2 = plist2->next;
                }
                ret.plist1 = plist1;
                ret.flag = 3;
                return ret;
            }
            else
            {
                //两个交点都在环上
                ret.plist1 = tmp1;
                ret.plist2 = tmp2;
                ret.flag = 3;
                return ret;
            }
        }
        else
        {
            //不相交
            ret.flag = 3;
            return ret;
        }
    }
    else
    {
        //一个带环，一个不带环。必不相交
        ret.flag = 2;
        return ret;
    }
}

参考