add quick sort which B:O(nlogn) A:O(nlogn) W:O(n*n)

sort/quicksort.go

@@ -0,0 +1,105 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"math/rand"
+)
+
+var (
+	a     []int = []int{15, 9, 8, 1, 4, 11, 7, 12, 13, 6, 5, 3, 16, 2, 10, 14}
+	DEBUG bool
+)
+
+func init() {
+	flag.BoolVar(&DEBUG, "debug", false, "no debug by default")
+}
+
+func swap(a []int, p1, p2 int) {
+	t := a[p1]
+	a[p1] = a[p2]
+	a[p2] = t
+}
+
+func partition(a []int, l, r, pivotIndex int) int {
+	if l > r || pivotIndex < l || pivotIndex > r {
+		panic("bad argument: out of index")
+	}
+
+	if l == r {
+		return l
+	}
+
+	if DEBUG {
+		log.Println("<partition:", a, l, r, pivotIndex)
+	}
+
+	pivot := a[pivotIndex]
+
+	//move pivot to the end of the array
+	swap(a, pivotIndex, r)
+
+	/* all values <= pivot are moved to front of array and pivot inserted just after them. */
+	store := l
+	for i := l; i < r; i++ {
+		if a[i] < pivot {
+			swap(a, i, store)
+			store++
+		}
+	}
+
+	swap(a, r, store)
+
+	if DEBUG {
+		log.Println(">partition:", a, store)
+	}
+
+	return store
+}
+
+/* select [l, r] */
+func selectPivotIndex(a []int, l, r int) int {
+	if l >= r {
+		return l
+	}
+
+	return l + rand.Intn(r-l)
+}
+
+func quickSort(a []int, l, r int) {
+	if DEBUG {
+		log.Println("quickSort:[D1]", a, l, r)
+	}
+
+	if l >= r {
+		return
+	}
+
+	pivotIndex := selectPivotIndex(a, l, r)
+	if DEBUG {
+		log.Println("quickSort:[D2]", pivotIndex)
+	}
+
+	pivotIndex = partition(a, l, r, pivotIndex)
+	if DEBUG {
+		log.Println("quickSort:[D3]", pivotIndex)
+	}
+
+	if l < pivotIndex-1 {
+		quickSort(a, l, pivotIndex-1)
+	}
+	if pivotIndex+1 < r {
+		quickSort(a, pivotIndex+1, r)
+	}
+}
+
+func main() {
+	flag.Parse()
+
+	fmt.Println(a, len(a))
+
+	quickSort(a, 0, len(a)-1)
+
+	fmt.Println(a)
+}