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) {
        ListNode* slow = pHead;
        ListNode* fast = pHead;
        
        while(fast != nullptr && fast->next != nullptr){
            slow = slow->next;
            fast = fast->next->next;
            if(slow == fast){
                break;
            }
        }
        if(fast == nullptr || (fast != nullptr && fast->next == nullptr)){
            return nullptr;
        }
        
        slow = pHead;
        while(slow != fast){
            slow = slow->next;
            fast = fast->next;
        }
        return slow;
        
    }
};