lintcode-103-带环链表 II

带环链表 II

思路

code

/**
* Definition of ListNode
* class ListNode {
* public:
*     int val;
*     ListNode *next;
*     ListNode(int val) {
*         this->val = val;
*         this->next = NULL;
*     }
* }
*/
class Solution {
public:
/**
* @param head: The first node of linked list.
* @return: The node where the cycle begins.
*           if there is no cycle, return null
*/
ListNode *detectCycle(ListNode *head) {
// write your code here
ListNode * fast = head, *slow = head, *result = NULL;

while(fast != NULL && fast->next != NULL) {
slow = slow->next;
fast = fast->next->next;

if(fast == slow) {
break;
}
}

if(fast != NULL && fast->next != NULL) {
fast = head;
while(fast != slow) {
slow = slow->next;
fast = fast->next;
}
result = fast;
}
return result;
}
};