Leetcode_linked-list-cycle-ii

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地址：http://oj.leetcode.com/problems/linked-list-cycle-ii/

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

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

Follow up: 不使用额外空间代码：

/**  * Definition for singly-linked list.  * struct ListNode {  *     int val;  *     ListNode *next;  *     ListNode(int x) : val(x), next(NULL) {}  * };  */ class Solution { private:     int Cnt_link_len(ListNode* head)     {         int cnt = 0;         while(head)         {             ++cnt;             head = head->next;         }         return cnt;     } public:     ListNode *detectCycle(ListNode *head) {         if(!head)             return NULL;         ListNode *fast = head->next, *slow = head;         while(fast && fast != slow)         {             fast = fast->next;             if(fast)                 fast = fast->next;             else                 return NULL;             slow = slow->next;         }         if(!fast)             return NULL;         fast = fast->next;         slow->next = NULL;         int len1 = Cnt_link_len(head);         int len2 = Cnt_link_len(fast);         if(len1<len2)             swap(head, fast);         len1 = len1>=len2 ? len1-len2 : len2-len1;         while(len1--)             head = head->next;         while(head && head != fast)         {             head = head->next;             fast = fast->next;         }         if(!head)             return NULL;         return head;     } };

参考代码：

/** * 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) {        map<ListNode*, int>lm;        while(head)        {            if(lm[head]==0)            {                lm[head] = 1;                head = head->next;            }            else            {                return head;            }        }        return NULL;    }};

SECOND TRIAL, unordered_map is better.

/** * 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) {        unordered_map<ListNode*, bool>mp;        while(head)        {            if(mp[head]==true)                return head;            mp[head] = true;            head = head->next;        }        return NULL;    }};

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