August 2017
Intermediate to advanced
360 pages
8h 35m
Chinese
Content preview from 算法技术手册（原书第2 版）
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44
｜
第
3
章
Graham 扫描算法小结
最好情况、平均情况和最坏情况：
O
(
n
log
n
)
graham(P)
low = point with lowest y coordinate in P
❶
remove low from P
sort P by descending polar angle with respect to low
❷
hull = {P[n-2], low}
❸
for i = 0 to n-1 do
while (isLeftTurn(secondLast(hull), last(hull), P[i])) do
remove last point in hull
❹
add P[i] to hull
remove duplicate last point
❺
return hull
❶
当多个点的
y
坐标值相同时，选择
x
坐标值最小的点。
❷
P
[0]
的极角最大，
P
[
n
－
2]
的极角最小。
❸
按照顺时针，从极角最小的点开始顺时针遍历。
❹
如果出现左拐，那么最后一个点必须被删除掉。
❺
因为这个重复点即是
P
[
n
－
2]
。
3.5.4 解决方案
如果手动计算凸包，你应该可以很轻松地处理好各种极端情况。但是如果需要用计算机
语言来描叙每一个步骤，我们可能会觉得比较困难。
Graham
扫描算法的关键点在于计
算和最低点的极角大小。一旦计算并且排序之后，算法只需要遍历所有的点，不断地构
建凸包并且根据新发现的信息调整结构即可。代码见例
3-1 ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.