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 *low = pHead;//慢指针
        ListNode *fast = pHead;//快指针
        ListNode *pre = nullptr;
        if(!pHead){
            return pre ;
        }
        while(fast->next  && fast->next->next )
        {
            fast = fast->next->next;
            low = low->next;
            if(fast == low)
            {
                ListNode *index1 = fast;
                ListNode *index2 = pHead;
                while(index1 != index2){
                    index1 = index1->next;
                    index2 = index2->next;
                }
                return index1;
            }

        }
        return pre;

    }
};