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VOA: standard.2009.12.18
Again, if you didn't hear it, the answer was it's log.
因为每次我可以把这个问题的规模,缩小一半,你答对了。
Now hopefully you're keeping a sheet of paper with you and you're writing down what you eat so you don't forget everything, and then you sit down at your computer at the end of the day and you log it all in.
希望你们可以随时带张纸,以便随时记下,吃的东西以防忘记,然后坐在电脑前,在一天结束时输入电脑
For example, this is x=t, x=t^, you're going to have x=sin t and cos t and log t.
例如,这是x=t,x=t^,你还会遇到正弦,余弦,对数函数
With this, if I can assume that accessing the i'th element of a list is constant, then you can't see that the rest of that analysis looks just like the log analysis I did before, and each step, no matter which branch I'm taking, I'm cutting the problem down in half.
读取数组中的第i个元素,是个常量时间的操作的话,我也就能像以前那样得到,这个算法是对数级复杂度的分析,并且每一步不管我选择哪个区间,我都可以把问题的规模缩小一半。
Once I have it sorted I can search it in log n time, but that's still isn't as good as just doing n. And this led to this idea of amortization, which is I need to not only factor in the cost, but how am I going to use it?
一旦对其完成排序,就可以在log,n的时间内对其完成搜索,但是这样做仍然不如n的复杂度,这样做引出了耗时分摊的想法,这时不仅需要考虑耗时的因素?
And then one of the things that I suggested was that if we could figure out some way to order it, and in particular, if we could order it in n log n time, and we still haven't done that, but if we could do that, then we said the complexity changed a little bit.
这就涉及到了排序,如果可以想出一种来将其进行排序,甚至可以在n,log,n的时间内完成,虽然目前我们没做这件事,但是一旦开始做这件事,那么复杂性就是发生一些变化。
And that's just a way of reminding you that we want to think carefully, but what are the things we're trying to measure when we talk about complexity here? It's both the size of the thing and how often are we going to use it? And there are some trade offs, but I still haven't said how I'm going to get an n log n sorting algorithm, and that's what I want to do today.
这只是在提醒你们我们要仔细的思考问题,但是当我们在讨论复杂性的时候,我们到底要衡量哪些东西?,是列表的大小和对其进行查找的频率吗?,这里面临一些取舍,但是我还没有说明,怎样得到一个n,log,n复杂度的排序算法,并且这是我今天想要讲的内容。
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