带环链表 II-LintCode，给定一个链表，如果链表中存在环，则返回到链表中环的起始节点的值，如果没有环，返回null。
样例
给出 -21->10->4->5, tail connects to node index 1，返回10

#ifndef C103_H
#define C103_H
#include
using namespace std;
class ListNode{
public:
    int val;
    ListNode *next;
    ListNode(int val){
        this->val = val;
        this->next = NULL;
    }
};
class Solution {
public:
    /**
    * @param head: The first node of linked list.
    * @return: The node where the cycle begins.
    *           if there is no cycle, return null
    */
    ListNode *detectCycle(ListNode *head) {
        // write your code here
        ListNode *fast = head, *slow = head;
        while (fast&&fast->next)
        {
            slow = slow->next;
            fast = fast->next->next;
            if (fast == slow)
                break;
        }
        if (fast==NULL || fast->next==NULL)
            return NULL;
        slow = head;
        while (slow != fast)
        {
            slow = slow->next;
            fast = fast->next;
        }
        return slow;
    }
};
#endif