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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，这一点在WPS下载最新地址中也有详细论述
`@receiver isNil ifTrue: `@nilBlock -> `@receiver ifNil: `@nilBlockIn fact, both versions work—but they apply different filters to the target node. Try to remember which one.
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