141. 环形链表（算法200题-19）

/**
 * 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 cur = head.next
  const map = new Map()
    while(cur) {
      if(map.has(cur)) {
        return true
      }
      map.set(cur)
      cur = cur.next
    }
    return false
};

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