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* head) {
        ListNode* slow = head;
        ListNode* fast = head;
        while(slow && fast) {
            slow = slow->next;
            if(fast->next) fast=fast->next->next;
            if(slow == fast) break;
        }
        if(fast == NULL || fast->next == NULL) return NULL;
        fast = head;
        while(fast != slow) {
            fast=fast->next;
            slow = slow->next;
        }
        return fast;
    }
};