A Mergesort algorithm for quick usage.

Mergesort.java

import java.util.Arrays;

public class Mergesort {
  public static void main(String[] args) {
      int[] set = { 3, 6, 1, 5, 12, 10 };

      System.out.println("Before: " + Arrays.toString(set));
      Mergesort.sort(set);
      System.out.println("After: " + Arrays.toString(set));
  }

  public static void sort(int[] set) {
    int[] tmp = new int[ set.length ];
    
    Mergesort.sort(set, tmp, 0, set.length);
  }

  public static void sort(int[] set, int[] tmp, int left, int right) {
    if (right - left < 2) {
      return;
    }   

    int middle = left + (right - left) / 2;
    
    Mergesort.sort(set, tmp, left, middle);
    Mergesort.sort(set, tmp, middle, right);
    Mergesort.merge(set, tmp, left, middle, right);
    Mergesort.copy(set, tmp, left, right);
  }

  public static void merge(int[] set, int[] tmp, int left, int middle, int right) {
    int i = left, j = middle;

    for (int k = left; k < right;  k++) {
      if (i < middle && (j >= right || set[i] <= set[j])) {
        tmp[k] = set[i];
        i++;
      }   
      else {
        tmp[k] = set[j];
        j++;
      }   
    }   
  }

  public static void copy(int[] a, int[] b, int left, int right) {
    for(int i = left; i < right; i++) {
      a[i] = b[i];
    }   
  }
}