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) {
        if (pHead == nullptr) return nullptr;
        
        // 快慢指针
        ListNode* pSlow = pHead->next;
        if (pSlow == nullptr)   return nullptr;

        ListNode* pFast = pHead->next->next;
        if (pFast == nullptr)   return nullptr;
        
        while (pFast != pSlow) {
            if (pFast->next == nullptr || pFast->next->next == nullptr) {
                return nullptr;
            }
            pFast = pFast->next->next;
            pSlow = pSlow->next;
        }
        pSlow = pHead;
        while (pSlow != pFast) {
            pSlow = pSlow->next;
            pFast = pFast->next;
        }
        return  pFast;

    }
};