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

/**
 * 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).