[LeetCode] 141. Linked List Cycle

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Problem

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:

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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.

141. Linked List Cycle

Example 2:

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Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.

141. Linked List Cycle

Example 3:

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Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

141. Linked List Cycle

Follow up:

Can you solve it using O(1) (i.e. constant) memory?

Explanation

  1. Both fast and slow pointers points to the head.
  2. When fast pointer moves two steps, slow pointer move one step.
  3. If fast pointer hits null, then no cycle; else if slow pointer and fast pointer point to the same node, then there’s cycle.

Solution

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/**
 * 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) {
        ListNode slow = head;
        ListNode fast = head;

        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (fast == null) break;
            if (slow == fast) return true;
        }

        return false;
    }
}