142. 环形链表 II

142. 环形链表 II

ListNode* fast =head;
ListNode* slow=head;
while(fast!=NULL&&fast->next!=NULL){
    slow=slow->next;
    fast=fast->next->next;
    if(slow==fast){
        ListNode* index1=fast;
        ListNode* index2=head;
        while(index1!=index2){
            index1=index1->next;
            index2=index2->next;
        }
        return inedx2;
    }
    return NULL;
}

/**
 * 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 *fast=head;
        ListNode *slow=head;
        while(fast!=NULL&&fast->next!=NULL){ 
            fast=fast->next->next;//走两部
            slow=slow->next;//走一步
            if(fast==slow){ //当快==慢时候(为什么不直返回slow呢？因为fast 不只走了一圈)
                ListNode* index1=fast;  //所以直接找它的地址再的地方
                ListNode* index2=head;  //从头数fast在哪
                while(index1!=index2){	
                    index1=index1->next;
                    index2=index2->next;
                }
                return index2;
            }
            
        }
        return NULL;
    }
};