LeetCode 141: Linked List Cycle

141. Linked List Cycle

Difficulty: Easy

Given a linked list, determine if it has a cycle in it.

Follow up:

Can you solve it without using extra space?

思路

定义两个指针，同时从链表头结点出发，一个指针一次走一步，另一个指针一次走两步。如果快的指针追上了慢的指针，存在环；如果快的指针追上之前就走到了链表末尾，不存在环。

代码

[C++]

class Solution {
public:
bool hasCycle(ListNode *head) {
if (head == NULL)
return false;
struct ListNode *slow = head->next;
if (slow == NULL)
return false;
struct ListNode *fast = slow->next;
while (fast != NULL && slow != NULL) {
if (fast == slow)
return true;
slow = slow->next;
fast = fast->next;
if (fast != NULL)
fast = fast->next;
}
return false;
}
};