141. 环形链表

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方案一: 哈希表

参考 160. 相交链表

方案二: 快慢指针 判圈算法 追及问题

var hasCycle = function(head) {
    if (head === null) return false
    let slow = head;
    let fast = head.next;

    while (fast !== null && fast.next !== null) {
        if (slow === fast) {
            return true;
        }
        slow = slow.next;
        fast = fast.next.next;
    }

    return false;
};