Data commit

Task/Quickselect-algorithm/Java/quickselect-algorithm.java

@@ -0,0 +1,53 @@
+import java.util.Random;
+
+public class QuickSelect {
+
+	private static <E extends Comparable<? super E>> int partition(E[] arr, int left, int right, int pivot) {
+		E pivotVal = arr[pivot];
+		swap(arr, pivot, right);
+		int storeIndex = left;
+		for (int i = left; i < right; i++) {
+			if (arr[i].compareTo(pivotVal) < 0) {
+				swap(arr, i, storeIndex);
+				storeIndex++;
+			}
+		}
+		swap(arr, right, storeIndex);
+		return storeIndex;
+	}
+	
+	private static <E extends Comparable<? super E>> E select(E[] arr, int n) {
+		int left = 0;
+		int right = arr.length - 1;
+		Random rand = new Random();
+		while (right >= left) {
+			int pivotIndex = partition(arr, left, right, rand.nextInt(right - left + 1) + left);
+			if (pivotIndex == n) {
+				return arr[pivotIndex];
+			} else if (pivotIndex < n) {
+				left = pivotIndex + 1;
+			} else {
+				right = pivotIndex - 1;
+			}
+		}
+		return null;
+	}
+	
+	private static void swap(Object[] arr, int i1, int i2) {
+		if (i1 != i2) {
+			Object temp = arr[i1];
+			arr[i1] = arr[i2];
+			arr[i2] = temp;
+		}
+	}
+	
+	public static void main(String[] args) {
+		for (int i = 0; i < 10; i++) {
+			Integer[] input = {9, 8, 7, 6, 5, 0, 1, 2, 3, 4};
+			System.out.print(select(input, i));
+			if (i < 9) System.out.print(", ");
+		}
+		System.out.println();
+	}
+
+}