141. 环形链表

题目

给定一个链表，判断链表中是否有环。

题解

哈希集合

/**
 * 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) return false;
  let s = new Set();
  s.add(head);
  let node = head.next;
  while (node) {
    if (s.has(node)) return true;
    s.add(node);
    node = node.next;
  }
  return false;
};

快慢指针

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