开始
开始

环形链表

TIP

用哈希表处理即可

141. 环形链表(简单)

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

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function (head) {
  const map = new Map();
  map.set(head, true);
  while (head) {
    const next = head.next;
    if (map.get(next)) {
      return true;
    } else {
      map.set(next, true);
      head = next;
    }
  }
  return false;
};

142. 环形链表 II(中等)

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

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var detectCycle = function (head) {
  const map = new Map();
  map.set(head, true);
  while (head) {
    const next = head.next;
    if (map.get(next)) {
      return next;
    }
    map.set(next, true);
    head = next;
  }
  return null;
};