Created
February 24, 2014 23:50
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Counting number of inversions using mergesort in Scala.
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// I commonly ask / get asked this question in interviews, | |
// but no candidate has ever used Scala so I thought I'd give it a try. | |
def inv(list : List[Int]) : Int = doInv(list)._1 | |
def doInv(list : List[Int]) : (Int, List[Int]) = | |
if (list.length <= 1) { | |
(0, list) | |
} else { | |
val (left, right) = list.splitAt(list.length / 2) | |
val (leftCount, leftList) = doInv(left) | |
val (rightCount, rightList) = doInv(right) | |
val (mergeCount, mergeList) = doMerge(leftList, rightList) | |
(leftCount + rightCount + mergeCount, mergeList) | |
} | |
def doMerge(left : List[Int], right : List[Int], count : Int = 0) : (Int, List[Int]) = | |
(left, right) match { | |
case (Nil, r) => (count, r) | |
case (l, Nil) => (count, l) | |
case (lhead :: ltail, rhead :: rtail) => | |
if (lhead <= rhead) { | |
val (lcount, list) = doMerge(ltail, right, count) | |
(count + lcount, lhead :: list) | |
} else { | |
val (rcount, list) = doMerge(left, rtail, count) | |
(count + left.length + rcount, rhead :: list) | |
} | |
} |
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@rubanm here's my version of it: https://github.com/feliperazeek/scala-playground/blob/master/src/main/scala/algo/CountInversions.scala. still need to improve, ran on a big dataset, ran outta memory.