C++/代码：

图片：

C++/代码：

class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        auto *i = pHead, *j = pHead;
        do {
            if (!j->next) return NULL; //
            i = i->next,j = j->next->next; //慢指针走一步，快指针走两步
        }while(i != j);
        for (i = pHead; i != j; i = i->next) j = j->next;
        return i;
    }
};