[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;
        }
    }
};