LeetCode 141. Linked List Cycle

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

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.

Example 3:

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

解析：
判断链表是否存在环，设置两个指针slow和fast，slow每次走一步，fast每次走两步。如果slow和fast相遇则存在环。
代码如下：
[cc lang=”C++”]
/**
* 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) {
if(head == NULL)
return false;
ListNode *fast = head;
ListNode *slow = head;
while(fast && fast->next)
{
slow = slow->next;
fast = fast->next->next;
if(fast == slow)
return true;
}
return false;
}
};
[/cc]
运行结果：