[C语言][LeetCode][142]Linked List Cycle II

题目

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?

标签

Linked List、Two Pointers

难度

中等

分析

题目意思是如果链表里面有回环，找出回环开始的位置，如果没有，返回null。我的做法是先判断链表是否有回环，判断回环的方法可以参照下文
http://blog.csdn.net/Timsley/article/details/51155365
如果找到回环，快指针指回表头，然后快慢指针继续走，快慢指针每次都是走一步，如果相遇，就找到这个指针，返回即可。

C代码实现

struct ListNode *detectCycle(struct ListNode *head) {    struct ListNode *fast, *slow;    bool fgHasCycle = false;    if(!head)        return NULL;    fast = head;    slow = head;    while(fast && fast->next)    {        fast = fast->next->next;        slow = slow->next;        if(fast == slow)        {            fgHasCycle = true;            break;        }    }    if(true == fgHasCycle)    {        fast = head;        while(fast)        {            if(fast == slow)                return fast;            fast = fast->next;            slow = slow->next;        }    }    else        return NULL;}