链表-带环链表 II-困难

描述
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，如果没有环，返回null。
您在真实的面试中是否遇到过这个题？  是
样例
给出 -21->10->4->5, tail connects to node index 1，返回10
挑战

不使用额外的空间

题目链接

程序

/**
 * Definition of ListNode
 * 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 * nodeCycle(ListNode *head1, ListNode *head2){
        while(head1 != head2){
            head1 = head1->next;
            head2 = head2->next;
        }
        return head1;
    }
    ListNode * detectCycle(ListNode * head) {
        // write your code here
        if(head == NULL || head->next == NULL)
            return NULL;
        ListNode *slow = head, *fast = head;
        while(fast != NULL && fast->next != NULL){
            slow = slow->next;
            fast = fast->next->next;
            if(fast == slow)
                return nodeCycle(head, slow);
        }
        return NULL;    
    }
};