题解 | #链表中环的入口结点#

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
  public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        if (pHead == nullptr) return nullptr;
        ListNode* fast = pHead, *slow = pHead;
        while (fast) {
		  //不存在环返回nullptr
            if (fast->next == nullptr || fast->next->next == nullptr) return nullptr;
            slow = slow->next;
            fast = fast->next->next;
            if (fast == slow) break;
        }
        fast = pHead;
        while (fast != slow) {
            fast = fast->next;
            slow = slow->next;
        }
        return slow;
    }
};

题解思路参考：https://zhuanlan.zhihu.com/p/103626709