Initial data commit

Task/K-d-tree/Java/k-d-tree-2.java

@@ -0,0 +1,52 @@
+import java.util.*;
+
+//
+// Java implementation of quickselect algorithm.
+// See https://en.wikipedia.org/wiki/Quickselect
+//
+public class QuickSelect {
+    private static final Random random = new Random();
+
+    public static <T> T select(List<T> list, int n, Comparator<? super T> cmp) {
+        return select(list, 0, list.size() - 1, n, cmp);
+    }
+
+    public static <T> T select(List<T> list, int left, int right, int n, Comparator<? super T> cmp) {
+        for (;;) {
+            if (left == right)
+                return list.get(left);
+            int pivot = pivotIndex(left, right);
+            pivot = partition(list, left, right, pivot, cmp);
+            if (n == pivot)
+                return list.get(n);
+            else if (n < pivot)
+                right = pivot - 1;
+            else
+                left = pivot + 1;
+        }
+    }
+
+    private static <T> int partition(List<T> list, int left, int right, int pivot, Comparator<? super T> cmp) {
+        T pivotValue = list.get(pivot);
+        swap(list, pivot, right);
+        int store = left;
+        for (int i = left; i < right; ++i) {
+            if (cmp.compare(list.get(i), pivotValue) < 0) {
+                swap(list, store, i);
+                ++store;
+            }
+        }
+        swap(list, right, store);
+        return store;
+    }
+
+    private static <T> void swap(List<T> list, int i, int j) {
+        T value = list.get(i);
+        list.set(i, list.get(j));
+        list.set(j, value);
+    }
+
+    private static int pivotIndex(int left, int right) {
+        return left + random.nextInt(right - left + 1);
+    }
+}