141. Linked List Cycle
Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
满足 without using extra space这个要求的话,可以只用2个指针来判断一个链表中的环,快的指针一次走2个结点,慢指针一次走一个结点,若有环路,两指针会先后进入环路,在慢指针被快指针“扣圈”时两指针在环路上相遇,代码:
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool hasCycle(ListNode *head) {
ListNode* faster, *slower;
if (head == NULL) return false;
faster = slower = head;
while (true) {
slower = slower->next;
faster = faster->next;
if (faster == NULL) return false;
faster = faster->next;
if (faster == NULL) return false;
if (slower == faster) return true;
}
}
};