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At its core, a stream is just a sequence of data that arrives over time. You don't have all of it at once. You process it incrementally as it becomes available.
，这一点在heLLoword翻译官方下载中也有详细论述
arr[j + 1] = arr[j]; // 元素后移
res[realIdx] = stack.length ? stack.at(-1) : -1;