В России ответили на имитирующие высадку на Украине учения НАТО18:04
思路：倒序单调栈。弹出所有 ≤ 当前身高的元素（这些人都能被看到），count 为弹出数量；若栈非空，还能看到栈顶（第一个更高的人），故 +1。能看到的人数 = count + (栈非空 ? 1 : 0)。
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If we think about this algebraically, what we really want to do is express the input pixel as the weighted sum of palette colours. This is nothing more than a linear combination of palette colours with weights :
// Signal how many bytes we wrote