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141.环形链表 ​

思路 ​

快慢指针，快指针每次走两步，慢指针每次走一步，如果有环就一定会相交

题解 ​

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

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function (head) {
  let fast = head;
  let slow = head;
  // while循环保证至少有两个节点
  while (fast && fast.next) {
    fast = fast.next.next;
    slow = slow.next;
    if (slow === fast) return true;
  }
  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) {
  let fast = head;
  let slow = head;
  // while循环保证至少有两个节点
  while (fast && fast.next) {
    fast = fast.next.next;
    slow = slow.next;
    if (slow === fast) return true;
  }
  return false;
};