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* pf = pHead, *ps = pHead;
        while(pf && pf->next)
        {
            pf = pf->next->next;
            ps = ps->next;
            if(pf == ps)
                break;
        }
        if(pf==nullptr || pf->next==nullptr) return nullptr;
        pf = pHead;
	  //在下一圈追上
        while(pf != ps)
        {
            pf = pf->next;
            ps = ps->next;
        }
        return ps;
    }
};