struct ListNode* EntryNodeOfLoop(struct ListNode* pHead ) {
    // write code here
if (pHead == NULL || pHead->next == NULL) {
        return NULL;
    }

struct ListNode* slow = pHead->next;
struct ListNode* fast = pHead->next->next;

while (fast != NULL && fast->next != NULL) {
        if (slow == fast) {
            // 链表中存在环
            slow = pHead;
            while (slow != fast) {
slow = slow->next;
fast = fast->next;
            }
            return slow;
        }
slow = slow->next;
fast = fast->next->next;
    }

    return NULL; // 链表中不存在环
}