[LeetCode] 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?

解题思路

使用快慢两个指针，如果快的指针赶上了慢的，则说明存在回路。

实现代码

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */

//Runtime:18 ms
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (head == NULL)
        {
            return false;
        }
        ListNode *slow = head;
        ListNode *fast = head;
        while (fast->next && fast->next->next)
        {
            slow = slow->next;
            fast = fast->next->next;
            if (fast == slow)
            {
                return true;
            }
        }

        return false;
    }
};