题解 | #链表中环的入口结点(优化)#

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
public:

    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        if(pHead == NULL){
            return NULL;
        }
        ListNode* fast = pHead;
        ListNode* slow = pHead;
        bool iscircle = false;
        while(fast!=NULL && fast->next!=NULL ){
            fast = fast->next->next;
            slow = slow->next;
            if(fast == slow){
                iscircle = true;
                fast = pHead;
                break;
            }
        }
        if(iscircle){
            while(fast!=slow){
                fast = fast->next;
                slow = slow->next;
            }
            return fast;
        }
        return NULL;

    }
};