142. 环形链表 II

 

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if(head==NULL)return NULL;
        //快慢指针
        ListNode* L=head;
        ListNode* R=head;
        
        //编译器默认R->next 用之前，必须判断下R是否为空
        while(R!=NULL&&R->next!=NULL)
        {
            L=L->next;
            R=R->next->next;
            if(L==R)
            {
                ListNode* start=head;
                while(start!=R)
                {
                    start=start->next;
                    R=R->next;
                }
                return start;
            }
            
        }
        return NULL;
    }
};