142环形链表II

/**
 * 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) {
        ListNode *cur1 = head ;//cur1 是快指针
        ListNode *cur2 = head;//cur2 是慢指针
        if(!head||!head->next)//如果头节点就是空，那就直接输出false
            return NULL;
        while(cur1&&cur1->next)
        {
            
            cur1 = cur1->next->next;
            cur2 = cur2->next;
			//相等的时候，我们进行快节点指向头部，慢节点不动，然后一起走的操作
            if(cur1==cur2)
            {
                cur1 = head;
                while(cur1!=cur2)
                {
                   cur1= cur1->next;
                   cur2=cur2->next;
                }
               return  cur2;
            }
        }
         return NULL;
    }
};