Add chapter 24 answers

ch_23/Exercise23_04.java

@@ -0,0 +1,92 @@
+package ch_23;
+
+import java.util.Arrays;
+
+/**
+ * 23.4 (Improve quick sort) The quick sort algorithm presented in the book selects the
+ * first element in the list as the pivot. Revise it by selecting the median among the
+ * first, middle, and last elements in the list.
+ */
+public class Exercise23_04 {
+    public static void quickSort(int[] list) {
+        /* Exercise 23.4 */
+        int medianIdx = indexOfMedian(list);
+        quickSort(list, medianIdx, list.length - 1);
+    }
+
+    public static void quickSort(int[] list, int first, int last) {
+        if (last > first) {
+            int pivotIndex = partition(list, first, last);
+            quickSort(list, first, pivotIndex - 1);
+            quickSort(list, pivotIndex + 1, last);
+        }
+    }
+
+    /**
+     * Partition the array list[first..last]
+     */
+    public static int partition(int[] list, int first, int last) {
+        int pivot = list[first]; // Choose the first element as the pivot
+        int low = first + 1; // Index for forward search
+        int high = last; // Index for backward search
+
+        while (high > low) {
+            // Search forward from left
+            while (low <= high && list[low] <= pivot)
+                low++;
+
+            // Search backward from right
+            while (low <= high && list[high] > pivot)
+                high--;
+
+            // Swap two elements in the list
+            if (high > low) {
+                int temp = list[high];
+                list[high] = list[low];
+                list[low] = temp;
+            }
+        }
+
+        while (high > first && list[high] >= pivot)
+            high--;
+
+        // Swap pivot with list[high]
+        if (pivot > list[high]) {
+            list[first] = list[high];
+            list[high] = pivot;
+            return high;
+        } else {
+            return first;
+        }
+    }
+
+    /**
+     *
+     * @param list int array to find median of first, middle and last nums
+     * @return the index of the median value
+     */
+    static int indexOfMedian(int[] list) {
+        int[] temp = {list[0], list[list.length / 2], list[list.length - 1]};
+        Arrays.sort(temp);
+        if (list[0] == temp[1]) {
+            return 0;
+        }
+        else if (list[list.length / 2] == temp[1]) {
+            return list.length / 2;
+        }
+
+        return list.length - 1;
+    }
+
+    /**
+     * A test method
+     */
+    public static void main(String[] args) {
+        int[] list = {2, 3, 2, 5, 6, 1, -2, 3, 14, 12};
+        quickSort(list);
+        for (int i = 0; i < list.length; i++)
+            System.out.print(list[i] + " ");
+    }
+}
+
+