public with sharing class Dataclass {
...
try {
        for(DarexSort__c sortobj : sortAdRec){
            String sortString = sortobj.Unsifie__c;
            List<String> sortList = sortString.split(',');
            List<Integer> intList = new List<Integer>();
            for(String s : sortList){
                intList.add(Integer.valueOf(s));
            }
            intList.sortStatic(); // Error here
            sortString = string.join(intList,',');
            sortobj.Sofie__c = sortString;
            sortRec.add(sortobj);
        }
        update sortRec;
        sortRec.clear();
...
}

public class Sortclass{
  private List<Integer> mNumbers;
  private Integer mNumber;
  public static void sortStatic(List<Integer> values){
        (new Quicksort()).sort(values);
  }
  public void sort(List<Integer> values) {
    if (values ==null || values.size()==0){
      return;
    }
    this.mNumbers = values;
    mNumber = values.size();
    quicksort(0, mNumber - 1);
  }
  private void quicksort(Integer low, Integer high) {
    Integer i = low, j = high;
    Integer pivot = mNumbers[low + (high-low)/2];
    while (i <= j) {
      while (mNumbers[i] < pivot) {
        i++;
      }
      while (mNumbers[j] > pivot) {
        j--;
      }
      if (i <= j) {
        exchange(i, j);
        i++;
        j--;
      }
    }
    // Recursion
    if (low < j)
      quicksort(low, j);
    if (i < high)
      quicksort(i, high);
  }
  private void exchange(Integer i, Integer j) {
    Integer temp = mNumbers[i];
    mNumbers[i] = mNumbers[j];
    mNumbers[j] = temp;
  }
}