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package sort
import (
"algorithms/ds"
)
// Quick3Way uses a 3-way partitioning so it is more efficient dealing with duplicates
func Quick3Way(a ds.ArrayList) ds.ArrayList {
Shuffle(a)
quick3Way(a)
return a
}
func quick3Way(a ds.ArrayList) {
length := len(a)
if length <= 10 {
Insertion(a)
return
}
lt := 0 // Lower than
i := 1 // lt..i contain duplicates
gt := length - 1 // Greater than
Insertion(a[0:3])
a.Swap(0, 1)
v := a[0] // Partitioning item
for i <= gt {
switch {
case a[i] < v:
a.Swap(lt, i)
lt++
i++
case a[i] > v:
a.Swap(i, gt)
gt--
default:
// Duplicate
i++
}
}
// Now a[lo..lt-1] < v = a[lt..gt] < a[gt+1..hi]
quick3Way(a[0:lt])
quick3Way(a[gt+1:])
}
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