在比较时注意==和equals的使用

public boolean isPail (ListNode<Integer> head) {
    ArrayList<Integer> list = new ArrayList<>();
    while (head != null) {
        list.add(head.val);
        head = head.next;
    }
    int size = list.size();
    for (int i = 0; i < size/2; i++) {
        if (!list.get(i).equals(list.get(size - 1 - i))) {
            return false;
        }
    }
    return true;
}