单链表是否有环及环入口点

参考与原理：https://www.jianshu.com/p/ef71e04241e4

https://blog.csdn.net/danny_idea/article/details/89504629

#include <bits/stdc++.h>
using namespace std;

typedef struct node pnode;
struct node
{
    int info;
    pnode *next;
};

void create(pnode **head)   //创建链表
{
    (*head) = (pnode *)malloc(sizeof(pnode));
    (*head)->info = 0;  //创建头节点，记录链表节点数
    (*head)->next = nullptr;
}

void add(pnode *head, int val)  //尾插法给链表添加节点
{
    head->info++;

    while (head->next != nullptr)
        head = head->next;

    pnode *temp = (pnode *)malloc(sizeof(pnode));
    temp->info = val;
    temp->next = nullptr;
    head->next = temp;
}

void show(pnode *head)  //打印链表
{
    printf("[%d] ", head->info);
    head = head->next;

    while (head != nullptr)
    {
        printf("%d ", head->info);
        head = head->next;
    }
}

void makeloop(pnode *head, int val)     //给链表建环，val为环入口，下标从1开始
{
    pnode *temp = head;

    while (val--)
        temp = temp->next;

    while (head->next != nullptr)
        head = head->next;

    head->next = temp;
}

void judgeloop(pnode *head) //判断链表是否有环
{
    pnode *fast, *slow;

    fast = slow = head;

    do
    {
        fast = fast->next;
        if (fast != nullptr)    //这里要记得判断一次是否为空
            fast = fast->next;

        slow = slow->next;
    } while (fast!=nullptr && fast!=slow);

    if (fast == slow)
        printf("有环
");

    else
        printf("无环
");
}

void findEntrance(pnode *head)  //找链表环入口，下标从1开始
{
    pnode *fast, *slow;

    fast = slow = head;

    do
    {
        fast = fast->next;
        if (fast != nullptr)
            fast = fast->next;

        slow = slow->next;
    } while (fast!=nullptr && fast!=slow);  //先使两指针相遇

    slow = head;
    int ind = 0;    //记录下标

    while (fast != slow)
    {
        slow = slow->next;
        fast = fast->next;
        ind++;
    }

    printf("入口节点下标：%d
", ind);
}

int main()
{
    pnode *head;

    create(&head);
    add(head, 1);
    add(head, 2);
    add(head, 3);
    add(head, 4);
    add(head, 5);
    add(head, 6);
    show(head);
    judgeloop(head);
    makeloop(head, 3);
    judgeloop(head);
    findEntrance(head);

    return 0;
}