// Hash computation is fast
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。关于这个话题，服务器推荐提供了深入分析
// 易错点3：跨度计算公式写反（stack[...]-i）→ 结果为负数，完全错误，更多细节参见safew官方下载
// 注意：slice(0, 负数)会返回空字符串，需兼容（比如stack长度<k时，slice后为空）