add generic quicksort

Author: ymzong

src/section21/GenericQuickSort.java

@@ -0,0 +1,57 @@
+package section21;
+
+import java.util.Arrays;
+import java.util.Comparator;
+
+/**
+ * This example demonstrates the syntax for declaring and using generic methods.
+ */
+public class GenericQuickSort {
+
+    /* Quicksorts the entire array using the given comparator */
+    public static <T> void quickSort(T[] arr, Comparator<T> cmp) {
+        quickSort(arr, cmp, 0, arr.length - 1);
+    }
+
+    /* Quicksorts the array using the given comparator between start and end index (inclusive) */
+    private static <T> void quickSort(T[] arr, Comparator<T> cmp, int start, int end) {
+        if (start >= end) {     // empty or singleton case
+            return;
+        }
+
+        // determine pivot value, start pointer, and end pointer
+        T pivot = arr[start + (end-start) / 2];
+        int i = start;
+        int j = end;
+
+        while (i <= j) {
+            while (cmp.compare(arr[i], pivot) < 0) { i++; }
+            while (cmp.compare(pivot, arr[j]) < 0) { j--; }
+
+            if (i <= j) {
+                T tmp = arr[i];
+                arr[i] = arr[j];
+                arr[j] = tmp;
+                i++;
+                j--;
+            }
+        }
+
+        quickSort(arr, cmp, start, j);
+        quickSort(arr, cmp, i, end);
+
+    }
+
+    public static void main(String[] argv) {
+        Comparator<Integer> desc = new Comparator<Integer>() {
+            public int compare(Integer x, Integer y) {
+                return y - x;
+            }
+        };
+
+        Integer[] arr = new Integer[] { 2, 19, 54, -14, 5, 19, 9351, 1024 };
+        quickSort(arr, desc);   // sort the array in descending order
+
+        System.out.println(Arrays.toString(arr));
+    }
+}