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 <cstddef>
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        ListNode LL = ListNode(-1);
        ListNode* PreHead = &LL;
        PreHead->next = pHead;

        ListNode* lower = PreHead, *faster = PreHead;

        while (faster->next) {

            faster = faster->next->next;

            lower = lower->next;

            if(!faster) return nullptr;


            if(faster == lower) break;

        }
        if(!faster->next) return nullptr;
        lower = PreHead;
        while (true) {
            lower = lower->next;
            faster = faster->next;
            if(lower == faster) break;

        }
        return lower;

        


    }
};