学步园

解法参考：http://blogread.cn/it/article/2452?f=hot1

先判断有没有环，有环的话，设两个指针，一个指针在head，另一个指针在相遇的点，两个指针同时往前走，则这两个指针相遇的点就是环的开始节点。

/**
 * 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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ListNode *cross = findCross(head);
        ListNode *tmp = head;
        if(cross != NULL){
            while(tmp != cross){
                cross = cross->next;
               tmp = tmp->next;
            }    
        }
        return cross;
    }
    ListNode *findCross(ListNode *head){
        if(head == NULL || head->next == NULL)    return NULL;
        ListNode *cross = NULL, *one = head->next, *two = head->next->next;
        while(one != NULL && two != NULL){
            if(one == two){
                cross = one;
                return cross;
            }
            if(two->next == NULL)    return cross;
            one = one->next;
            two = two->next->next;
        }
        return cross;
    }
};