LinkedListCycle.java

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 */

// 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.

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
  public boolean hasCycle(ListNode head) {
    if (head == null || head.next == null) { return false; }

    ListNode slow = head, fast = head.next;
    while (fast != null) {
      if (slow == fast) { return true; }

      slow = slow.next;
      if (fast.next != null) {
        fast = fast.next.next;
      } else {
        return false;
      }
    }

    return fast == null ? false : true;
  }
}