From deaa4e1c33cd2c1c75f698881918688055abfa51 Mon Sep 17 00:00:00 2001
From: Paul Buetow <pbuetow@mimecast.com>
Date: Fri, 7 Aug 2020 11:14:35 +0100
Subject: add quick2

---
 sort/quick2.go | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)
 create mode 100644 sort/quick2.go

(limited to 'sort/quick2.go')

diff --git a/sort/quick2.go b/sort/quick2.go
new file mode 100644
index 0000000..688e410
--- /dev/null
+++ b/sort/quick2.go
@@ -0,0 +1,43 @@
+package sort
+
+import (
+	"algorithms/ds"
+)
+
+// Quick2 uses a 3-way partitioning so it is more efficient
+// dealing with duplicates
+func Quick2(a ds.ArrayList) ds.ArrayList {
+	a = Shuffle(a)
+	quick2(a, 0, len(a)-1)
+	return a
+}
+
+func quick2(a ds.ArrayList, lo, hi int) {
+	if hi <= lo {
+		return
+	}
+
+	lt := lo    // Lower than
+	i := lo + 1 // lt..i contain duplicates
+	gt := hi    // Greater than
+	v := a[lo]  // Partitioning item
+
+	for i <= gt {
+		switch a[i].Compare(v) {
+		case -1:
+			a.Swap(lt, i)
+			lt++
+			i++
+		case 1:
+			a.Swap(i, gt)
+			gt--
+		default:
+			// Duplicate
+			i++
+		}
+	}
+	// Now a[lo..lt-1] < v = a[lt..gt] < a[gt+1..hi]
+
+	quick2(a, lo, lt-1)
+	quick2(a, gt+1, hi)
+}
-- 
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