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) {
    }
};
*/
#include <ios>
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        if (!pHead || !(pHead->next)) return nullptr;
        ListNode* slow = pHead;
        ListNode* fast = pHead;

        do {
        slow = slow->next;
        if (!slow) return nullptr;
        fast = fast->next;
        if (!fast) return nullptr;
        fast = fast->next;
        if (!fast) return nullptr;
        }while (slow!=fast);

        ListNode* mid = pHead;
        while (mid != slow)
        {
            mid=mid->next;
            slow=slow->next;
        }
        return slow;
    }
};