Interesting, How to find next greater number with same digits

Given a number N, find the smallest number that has same set of digits as N and is greater than N. If theres no number greater than N by same same set of digits then print -1.
For example if N = 354 then all possible cobinations of the N digits will be 345, 435, 453, 534, 543. So the output will be 435 is the next smallest of all combinations and greater than N.

Few examples:

N = 354
Output: 435

N = 218765
Output: 251678

N = 1234
Output: 1243

N = 4321
Output: -1

N = 534976
Output: 536479

Below code a simple permutation and combination of all digits from given N and get the next number from given number. If don't find any number then return -1.

For permutation and combination we use recursive algorithm and stores the combination in the TreeSet. 

import java.util.TreeSet;

public class HighestNumber {

 private TreeSet<Integer> combination = new TreeSet<Integer>();

 public static void main(String[] args) {

  Integer n = 354;
  HighestNumber obj = new HighestNumber();
  int nxtNumber = obj.findNextHighest(n);
  System.out.println(nxtNumber);
 }

 private int findNextHighest(int n) {
  permutation("", Integer.toString(n));
  return getNextHighestNumber(n);
 }

 /*
  * Recursive method to get all combination of digits
  */
 private void permutation(String prefix, String str) {
  int len = str.length();
  if (len == 0) {
   combination.add(Integer.valueOf(prefix));
  } else {
   for (int i = 0; i < len; i++) {

    String p = prefix + str.charAt(i);
    String s = str.substring(0, i) + str.substring(i + 1, len);
    permutation(p, s);
   }
  }
 }

 /*
  * Get the next number from combination collection TreeSet
  */
 private int getNextHighestNumber(int n) {
  boolean flag = false;
  int nxtNumber = -1;
  for (Integer num : combination) {
   if (num == n)
    flag = true;
   else if (flag) {
    nxtNumber = num;
    break;
   }
  }
  return nxtNumber;
 }
}

OUTPUT:

435