投票算法

投票算法

多数表决

graph LR A(votes)--> B[[count]] --> C[[sort]] --> D[[last]] --> E(winner)

投票选择制

1. 淘汰第一权重票数最少的
2. 过滤淘汰者, 自动补位
3. 重复 1 2 直至 winner

graph LR A(ballots) --> B{只剩一人?} --Yes--> E(winner) B --No-->C[[淘汰第一权重票最少者]] --> D[[清除淘汰者票]] -->B

验证

> print (winner votes)
"Blue"

> print (winner' ballots)
"Green"

代码

import Data.List

-- 多数表决
votes :: [String]
votes = ["Red", "Blue", "Green", "Blue", "Blue", "Red"]

count :: Eq a => a -> [a] -> Int
count x = length . filter (== x)

result :: Ord a => [a] -> [(Int,a)]		-- 计数结果
result vs = sort [(count v vs, v) | v <- nub vs]

winner :: Ord a => [a] -> a
winner = snd . last . result


-- 选择投票制
ballots :: [[String]]
ballots = [["Red", "Green"],
            ["Blue"],
            ["Green", "Red", "Blue"],
            ["Blue", "Green", "Red"],
            ["Green"]]

elim :: Eq a => a -> [[a]] -> [[a]]
elim x = map (filter (/= x))

rank :: Ord a => [[a]] -> [a]	-- 候选人排序
rank = map snd . result . map head

winner' :: Ord a => [[a]] -> a
winner' bss = case rank (filter (/=[]) bss) of
                [c]     -> c
                (c:cs)  -> winner' (elim c bss)

参考: Programming in Haskell ch7.7