public int maxScore(int[] cardPoints, int k) {
    int maxWindow = 0, length = cardPoints.length;
    //先统计前k个元素的和，也是窗口内元素的和
    for (int i = 0; i < k; i++)
        maxWindow += cardPoints[i];
    //然后窗口移动，更新当前窗口的值
    int curWindow = maxWindow;
    for (int i = length - 1; i >= length - k; i--) {
        //窗口移动的时候一个元素会出窗口，一个元素会进入窗口。
        //cardPoints[k - (length - i)]是移除窗口的元素
        curWindow -= cardPoints[k - (length - i)];
        //cardPoints[i]是进入窗口的元素
        curWindow += cardPoints[i];
        //记录窗口的最大值
        maxWindow = Math.max(maxWindow, curWindow);
    }
    return maxWindow;
}