ch 23 ex 4

Author: Harry Dulaney

ch_23/Exercise23_04.java

@@ -0,0 +1,91 @@
+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 main(String[] args) {
+        int[] list = {2, 3, 2, 5, 6, 1, -2, 3, 14, 12};
+        quickSort(list);
+        System.out.println("Sorted List: \n" + Arrays.toString(list));
+    }
+
+    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);
+        }
+    }
+
+    public static void quickSort(int[] list) {
+        quickSort(list, 0, list.length - 1);
+    }
+
+    /**
+     * Partition the array list[first..last]
+     */
+    public static int partition(int[] list, int first, int last) {
+        /* Exercise 23_04 */
+        int pivotIdx = findMedian(list, first, last);
+        int pivot = list[pivotIdx];
+        int low = first; // 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[pivotIdx] = list[high];
+            list[high] = pivot;
+            return high;
+        } else {
+            return pivotIdx;
+        }
+    }
+
+    /**
+     * Exercise 23.04 solution
+     *
+     * @param list  current list
+     * @param first current left index
+     * @param last  current right index
+     * @return index of the median between first, last, and mid index values
+     */
+    static int findMedian(int[] list, int first, int last) {
+        int midIdx = (first + last) / 2;
+        int firstVal = list[first];
+        int lastVal = list[last];
+        int midVal = list[midIdx];
+        int max = Math.max(Math.max(firstVal, lastVal), midVal);
+        int min = Math.min(Math.min(firstVal, lastVal), midVal);
+        if (midVal != min && midVal != max) return midIdx;
+        else if (lastVal != min && lastVal != max) return last;
+        else return first;
+
+    }
+}
+
+