Data commit

Task/Quickselect-algorithm/Go/quickselect-algorithm-2.go

@@ -0,0 +1,55 @@
+package main
+
+import (
+    "fmt"
+    "sort"
+    "math/rand"
+)
+
+func partition(a sort.Interface, first int, last int, pivotIndex int) int {
+    a.Swap(first, pivotIndex) // move it to beginning
+    left := first+1
+    right := last
+    for left <= right {
+        for left <= last && a.Less(left, first) {
+            left++
+        }
+        for right >= first && a.Less(first, right) {
+            right--
+        }
+        if left <= right {
+            a.Swap(left, right)
+            left++
+            right--
+        }
+    }
+    a.Swap(first, right) // swap into right place
+    return right
+}
+
+func quickselect(a sort.Interface, n int) int {
+    first := 0
+    last := a.Len()-1
+    for {
+        pivotIndex := partition(a, first, last,
+	                        rand.Intn(last - first + 1) + first)
+        if n == pivotIndex {
+            return pivotIndex
+        } else if n < pivotIndex {
+            last = pivotIndex-1
+        } else {
+            first = pivotIndex+1
+        }
+    }
+    panic("bad index")
+}
+
+func main() {
+    for i := 0; ; i++ {
+        v := []int{9, 8, 7, 6, 5, 0, 1, 2, 3, 4}
+        if i == len(v) {
+            return
+        }
+        fmt.Println(v[quickselect(sort.IntSlice(v), i)])
+    }
+}