141. Linked List Cycle

Problem

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

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

Solution (Java)

/**
 * 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) {
            return false;
        }
        ListNode fast = head.next;
        ListNode slow = head;
        while (fast != null && fast.next != null) {
            if (fast == slow) {
                return true;
            }
            fast = fast.next.next;
            slow = slow.next;
        }
        return false;
    }
}

Solution (Javascript)

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
  var slow = head;
  var fast = head;
  while (slow && fast) {
    slow = slow.next;
    fast = fast.next ? fast.next.next : undefined;
    if (slow === fast) return true;
  }
  return false;
};

Explain:

fast 每次走两步，slow 每次走一步，如果链表有环的话，它们最终会相遇。

Complexity:

• Time complexity : O(n).
• Space complexity : O(1).