list - Linked List Cycle

一，Linked List Cycle

题目描述

Given a linked list, return the node where the cycle begins. If there is no cycle, returnnull.

我的代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */

/**
*  http://www.cnblogs.com/hiddenfox/p/3408931.html
*  重点是看它里面的方法三，介绍的结论。相似的问题有：
*  1，如果有环，求环的长度。
*/
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if(head == NULL || head->next == NULL){
            return NULL; 
        }  
        ListNode* fast = head;  
        ListNode* slow = head;  
        while(fast->next != NULL && fast->next->next != NULL){  
           slow = slow->next;  
           fast = fast->next->next;  
           if(slow == fast){  
               slow = head;  
               while(slow != fast){  
                   slow = slow->next;  
                   fast = fast->next;  
               }  
               return slow;  
           }  
       }  
       return NULL;  
    }
};