栈（stack）是先进后出的数据结构。单调栈在入栈时通过一定规则保持栈内元素有序（单调递增或单调递减），从而把「找左侧/右侧第一个更大/更小元素」从 O(n²) 降到 O(n)。
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需要索引（如算距离、下标）：栈存索引，用 nums[stack.at(-1)] 取值比较。
The reason is always the same: the content and the key that decrypts it are both present on the client’s machine. The user’s hardware decrypts the content to display it. The user’s hardware is, definitionally, something the user controls. Any sufficiently motivated person with the right tools can intercept the decrypted output.