JavaAlgorithms/Sorts/MergeSort.java

package Sorts;

/**
 * Generic merge sort algorithm.
 *
 * @see SortAlgorithm
 */
class MergeSort implements SortAlgorithm {

  /**
   * Generic merge sort algorithm implements.
   *
   * @param unsorted the array which should be sorted.
   * @param <T> Comparable class.
   * @return sorted array.
   */
  @Override
  public <T extends Comparable<T>> T[] sort(T[] unsorted) {
    doSort(unsorted, 0, unsorted.length - 1);
    return unsorted;
  }

  /**
   * @param arr the array to be sorted.
   * @param left the first index of the array.
   * @param right the last index of the array.
   */
  private static <T extends Comparable<T>> void doSort(T[] arr, int left, int right) {
    if (left < right) {
      int mid = (left + right) >>> 1;
      doSort(arr, left, mid);
      doSort(arr, mid + 1, right);
      merge(arr, left, mid, right);
    }
  }

  /**
   * Merges two parts of an array.
   *
   * @param arr the array to be merged.
   * @param left the first index of the array.
   * @param mid the middle index of the array.
   * @param right the last index of the array merges two parts of an array in increasing order.
   */
  private static <T extends Comparable<T>> void merge(T[] arr, int left, int mid, int right) {
    int length = right - left + 1;
    @SuppressWarnings("unchecked")
    T[] temp = (T[]) new Comparable[length];
    int i = left;
    int j = mid + 1;
    int k = 0;

    while (i <= mid && j <= right) {
      if (arr[i].compareTo(arr[j]) <= 0) {
        temp[k++] = arr[i++];
      } else {
        temp[k++] = arr[j++];
      }
    }

    while (i <= mid) {
      temp[k++] = arr[i++];
    }

    while (j <= right) {
      temp[k++] = arr[j++];
    }

    System.arraycopy(temp, 0, arr, left, length);
  }

  /** Driver code */
  public static void main(String[] args) {
    MergeSort mergeSort = new MergeSort();

    Integer[] arr = {4, 23, 6, 78, 1, 54, 231, 9, 12};
    mergeSort.sort(arr);
    for (int i = 0; i < arr.length - 1; ++i) {
      assert arr[i] <= arr[i + 1];
    }

    String[] stringArray = {"c", "a", "e", "b", "d"};
    mergeSort.sort(stringArray);
    for (int i = 0; i < stringArray.length - 1; ++i) {
      assert arr[i].compareTo(arr[i + 1]) <= 0;
    }
  }
}