题目

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?

代码

使用hashmap版

/**
* 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) {
std::map<ListNode *, bool> ifNodeOccured;
ListNode *p = head;
while ( p )
{
if ( ifNodeOccured.find(p) != ifNodeOccured.end() ) return p;
ifNodeOccured.insert(std::pair<ListNode *, bool>(p,true));
p = p->next;
}
return NULL;
}
};

不用hashmap版(O(1)空间复杂度)

/**
* 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) return false;
ListNode *p1 = head;
ListNode *p2 = head;
while ( p2->next && p2->next->next )
{
p2 = p2->next->next;
p1 = p1->next;
if (p2==p1)
{
p1 = head;
while (p1 != p2)
{
p1 = p1->next;
p2 = p2->next;
}
return p1;
}
}
return false;
}
};

Tips

个人还是喜欢hashmap的解法,因为比较统一;但当数据量很大的时候,开辟hashmap占用的资源也是蛮多的。

第二种解法参考的如下两篇日志:

http://www.cnblogs.com/hiddenfox/p/3408931.html

http://blog.csdn.net/cs_guoxiaozhu/article/details/14209743

这里只讨论有环的case

快慢指针技巧;慢==快,则快走的路程是慢走的路程的2倍;再让慢从头走,快从相遇点走,但每次改走一步,则再次相遇的地方就是起点。

这个无论是head到相遇点多长,都满足,算是一个特殊的技巧吧。还是很喜欢hashmap的做法。

====================================================

第二次过这道题,大体思路还记得。hashmap的方法一次写出来AC了,特殊技巧的写法参照了之前的代码。

/**
* 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) {
if (!head) return head;
ListNode* p1 = head;
ListNode* p2 = head;
while (p2)
{
p1 = p1->next;
p2 = p2->next;
if (p2)
{
p2 = p2->next;
}
else
{
return NULL;
}
if ( p1==p2 )
{
p1 = head;
while (p1!=p2)
{
p1 = p1->next;
p2 = p2->next;
}
return p1;
}
}
return NULL;
}
};

这里有个细节,需要记住:当p1=p2的时候,重置p1=head;接下来先要判断p1!=p2然后再开始走,否则,当整个链表是个环的时候,就出bug了。

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