Linked List Cycle II

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

Note: Do not modify the linked list.

Follow up:
Can you solve it without using extra space?

方法：寻找进入环的第一个节点。
参考https://leetcode.com/problems/linked-list-cycle-ii/。

/** * 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) {        if(!head||!head->next)            return NULL;        ListNode* slow = head,*fast = head;        bool first = true;        while(first || slow != fast){          //  ++step;             first = false;            if(slow->next)                slow = slow -> next;            else                return NULL;            if(fast->next && fast->next->next)                fast  = fast -> next -> next;            else                return NULL;        }        ListNode *cur = head;        while(cur!=slow){            cur = cur -> next;            slow = slow->next;        }        return cur;    }};