var removeNthFromEnd = function(head, n) {
    if (!head) return null;
    // 建立哈希表
    const m = new Map();
    // 让临时指针指向head
    let temp = head;
    let i = 1;
    // 循环开始
    while (temp) {
        m.set(i,temp);
        temp = temp.next;
        i++;
    }
    const x = i - n;
    if (x > 1) {
        m.get(x-1).next = m.get(x+1) || null;
    } else {
        return head.next;
    }
    return head
};