141 Linked List Cycle

141. Linked List Cycle

1. Question

Given a linked list, determine if it has a cycle in it.

Follow up: Can you solve it without using extra space?

2. Implementation

/**
 * 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, fast = head;

        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;

            if (fast == slow) {
                return true;
            }
        }
        return false;
    }
}

3. Time & Space Complexity

时间复杂度: O(n), 空间复杂度: O(1)