ShellSort.java Add shell sort algorithm implementation in Java

shell_sort/ShellSort.java

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+import java.util.Arrays;
+
+public class ShellSort {
+
+    // Driver method for testing sort algorithm
+    public static void main(String[] args) {
+        int[] array = {21, 3, 82, 12, 93, 56, 74, 38};
+        System.out.println("Before sort: " + Arrays.toString(array));
+        shellSort(array);
+        System.out.println("After sort: " + Arrays.toString(array));
+    }
+
+    /**
+     *
+     * Sorts an array by implementing the shell sort algorithm.
+     * Shell sort is an in-place comparison sort
+     * <p>
+     * Average case = depends on the gap<br>
+     * Worst case = O(n * log^2 n)<br>
+     * Best case = O(n)<br>
+     *
+     * @param array an unsorted array
+     */
+    public static void shellSort(int[] array) {
+        int inner, outer;
+        int temp;
+
+        int h = 1;
+        while (h <= array.length / 3) {
+            h = h * 3 + 1;
+        }
+        while (h > 0) {
+            for (outer = h; outer < array.length; outer++) {
+                temp = array[outer];
+                inner = outer;
+
+                while (inner > h - 1 && array[inner - h] >= temp) {
+                    array[inner] = array[inner - h];
+                    inner -= h;
+                }
+                array[inner] = temp;
+            }
+            h = (h - 1) / 3;
+        }
+    }
+}