0141_linked_list_cycle.cc

/**
 * Author: Lanqing Ye
 * Date: 2019-10-02
 */
/* 

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.


*/ 

/**
 * 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 || head -> next == nullptr) return false;
        ListNode *fast = head;
        ListNode *slow = head;
        while(fast -> next != nullptr && fast -> next -> next != nullptr){
            fast = fast -> next -> next;
            slow = slow -> next;
            if(fast == slow)    return true;
        }
        return false;
    }
};