Leetcode: Linked List Cycle II

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

参考了http://blog.csdn.net/whuwangyi/article/details/14103993

/**
* 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==NULL) return NULL;
ListNode* fastHead=head;
ListNode* slowHead=head;
// get the first meeting point
while(true){
slowHead=slowHead->next;
fastHead=fastHead->next;
if(fastHead==NULL) return NULL;
fastHead=fastHead->next;
if(fastHead==NULL) return NULL;
if(fastHead==slowHead) break;
}
// get the length of the cycle
int cnt=1;
slowHead=slowHead->next;
while(slowHead!=fastHead){
slowHead=slowHead->next;
cnt++;
}
// start from the head and make the fastHead ahead of slowHead by cnt nodes
fastHead=slowHead=head;
while(cnt!=0){
fastHead=fastHead->next;
cnt--;
}
while(fastHead!=slowHead){
fastHead=fastHead->next;
slowHead=slowHead->next;
}
return fastHead;
}
};