怎么判断链表有环，怎么找环节点

一、快慢指针法（判断是否有环）

int has_cycle(node *head) {
    if (head == NULL)  return false;
    node* fast = head;
    node* slow = head;

    while (1)
    {
        if (slow->next != NULL)  slow = slow->next;        //慢指针走一步
        else   return false;
        if (fast->next != NULL && fast->next->next != NULL)  fast = fast->next->next;  //快指针走两步
        else  return false;
        if (slow == fast)  return true;
    }
}