[Algorithm] 11. Linked List Cycle

Description

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

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.

Challenge

Follow up:
Can you solve it without using extra space? (O(1) (i.e. constant) memory)?

Solution

 /**
  * 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(true){
             // If a node is NULL, there is no cycle.
             if(fast->next == NULL || fast->next->next == NULL)  return false;

             slow = slow->next;
             fast = fast->next->next;

             // When the fast and the slow run into the same node, there's a cycle.
             if ( slow->val == fast->val )
                 return true;
         }
     }
 };