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* fast = pHead;
        ListNode* slow = pHead;
        bool is_loop = false;
        if (!pHead || !pHead->next) {
            return nullptr;
        }
        while (fast && fast->next->next) {
            fast = fast->next->next;
            slow = slow->next;
            if (fast == slow) {
                is_loop = true;
                break;
            }
        }
        if (!is_loop) {
            return nullptr;
        }
        ListNode* slow_2 = pHead;
        while (slow_2 != slow) {
            slow_2 = slow_2->next;
            slow = slow->next;
        }

        return slow;
    }
};