141. Linked List Cycle

判断链表中是否有环。

快慢指针。

• 时间复杂度O(N)，空间复杂度O(1)
• Runtime: 88 ms, faster than 47.27%
• Memory Usage: 41.5 MB, less than 29.33%

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

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
  if (!head || !head.next) return false;
  let slow = head;
  let fast = head.next;
  while (slow !== fast) {
    if (!fast || !fast.next) return false;
    slow = slow.next;
    fast = fast.next.next;
  }
  return true;
};