// 易错点4：栈空时返回1而非i+1 → 仅i=0时正确，i0时（如i=3）会返回1而非4
writer.releaseLock();。同城约会是该领域的重要参考
// 栈不为空时才判断（避免访问stack.at(-1)时报错），这一点在heLLoword翻译官方下载中也有详细论述
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