=》 https://www.acwing.com/problem/content/86/

=》题解 https://www.acwing.com/solution/content/741/

class Solution {
public:
    ListNode *entryNodeOfLoop(ListNode *head) {
        auto i = head, j = head;

        while(i != nullptr && j != nullptr)
        {
            i = i->next;
            j = j->next;
            if(j)
            {
              j = j->next;
              if(j == nullptr) return nullptr;
            }

            if(i == j && i && j) //保证i和j不相等
            {
                i = head;
                while(i != j)
                {
                    i = i->next;
                    j = j->next;
                }
                return i;
            }
        }
        return nullptr;
    }
};