Least Possible Number After Removing k digits

Problem

Please get the least number after deleting k digits from the input number. For example, if the input number is 24635, the least number is 23 after deleting 3 digits.

Solution

public class GetLeastkdigits {

 int getLeast(int num, int k)
 {
  ArrayList a = new ArrayList();
  HashMap map = new HashMap();
  if(num == 0) {
   a.add(0); 
   map.put(0, 0);
  }
  int index=0;
  while(num > 0)
  {
   a.add(num%10);
   map.put(num%10, index++);
   num/=10;
  }

  int retainCnt = a.size() - k;
  if(retainCnt <= 0) return -1;
  
  ArrayList copy = new ArrayList(a);
  Collections.sort(a);
  
  int[] pos = new int[retainCnt];
  for(int i= 0; i< retainCnt; i++)
  {
   pos[i] = map.get(a.get(i));
  }
  Arrays.sort(pos);

  StringBuffer s = new StringBuffer();
  for(int i= retainCnt-1; i>= 0; i--)
  {
   s.append(copy.get(pos[i]));
  }
  
  return Integer.valueOf(s.toString());
 }
 
 public static void main(String[] args) {
  GetLeastkdigits ob = new GetLeastkdigits();
  System.out.println(ob.getLeast(24635, 3));
 }