Rearrange a string so that all same characters become d distance away

// Steps to rearrange the string:
// - define array with size 256 which contains frequency of char index.
// - define heap node with key of type char and its frequency of type int
// - create max heap as per frequency element heap node
// - Itreate till length of string
//    remove max frequency node from heap
//    place it at k = i + (f-1)*d positions of result string


static String rearrange(String str, int d) throws CustomException{
int n = str.length();
if(n == 0 || n == 1) return str;
char[] res = new char[n];
for(int i=0; i {
res[i] = 0;
}

int[] arrf = new int[260];

heapnd[] hn = new heapnd[n];

  for (int i = 0; i {
arrf[str.charAt(i)] ++;
}
  int j =0;
  for(int i = 0; i<260 i="" p="">  {
  if(arrf[i] != 0)
  {
  hn[j++] = new heapnd((char) i ,arrf[i]);
  }
  }
 
  maxHeap(hn,j);
 
  for(int i =0; i  {
  if(res[i] != 0)
  {
  continue;
  }
heapnd maxfreq = hn[0];
int k;
for(int l = 1; l<= maxfreq.freq; l++)
{
k = i + (l-1)*d;
if(k>n-1) throw new CustomException("Can't rearrange this string.");
res[k] = maxfreq.key;
}
if(j > 0)
{
hn[0] = hn[j-1];
j = j-1;
maxheapify(hn, 0, j);
}
  }
 
  return new String(res);
 
 
}

private static void maxHeap(heapnd[] hn, int n){
if(n == 0 || n == 1) return;

for(int i = (n-1)/2; i>=0; i--)
{
maxheapify(hn, i, n);
}
}


private static void maxheapify(heapnd[] hn, int i, int n) {

int l = left(i);
int r = right(i);
int maxIndex = i;
if(l maxIndex = l;
if(r maxIndex = r;
if(maxIndex != i)
{
swap(hn, i, maxIndex);
}

}

private static void swap(heapnd[] hn, int i, int maxIndex2) {
heapnd tmp = hn[i];
hn[i] = hn[maxIndex2];
hn[maxIndex2] = tmp;
}

class CustomException extends Exception{

private static final long serialVersionUID = 1L;
private String str;

public CustomException(String str)
{
this.str = str;
}

public String toString()
{
return str;
}
}

Reference:

http://www.geeksforgeeks.org/rearrange-a-string-so-that-all-same-characters-become-at-least-d-distance-away/