【手撕代码1】-判断单链表是否有环+求环的长度+求入环点

核心思想：快慢指针

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

struct listnode {
    int val;
    listnode* next;
};
listnode* meet;
listnode* head;
//判断是否有环
bool panduan(listnode * head)
{
    if (head == NULL)return false;
    listnode* fast=head;
    listnode* slow=head;
    //因为这里fast有两个next，所以要保证fast->next也不为空
    while (fast != NULL&&fast->next!=NULL)
    {
        if (fast == slow)
        {
            meet = fast;
            return true;
        }
        fast = fast->next->next;
        slow = slow->next;
    }
    return true;
}
//求环长度
int len()
{
    listnode* p = meet;
    int sum = 1;
    p = p->next;
    while (p != meet)
    {
        sum++;
        p = p->next;
    }
    return sum;
}
//求入环点 L=kc-n
//只需要找到相遇节点，再让一个指针从头开始走即可
listnode* findnode()
{
    listnode* a=meet;
    listnode* b = head;;
    while (a && b)
    {
        if (a == b)return a;
        a = a->next;
        b = b->next;
    }
    return NULL;
}

int main()
{
    
    /*数据输入*/
    panduan(head);
    len();
    findnode();
    return 0;
}

具体详见 https://blog.csdn.net/etalien_/article/details/90272899