NextSmallerNumber2.java

/**
 *
 * @author Maikel Chandika (mkdika@gmail.com)
 */
public class NextSmallerNumber2 {

    public static void main(String[] args) {
        System.out.println(nextSmaller(21)); //12
        System.out.println(nextSmaller(531)); //513
        System.out.println(nextSmaller(907)); //790
        System.out.println(nextSmaller(2071)); //2017
        System.out.println(nextSmaller(544123)); // 543421
        System.out.println(nextSmaller(315)); // 153
        System.out.println(nextSmaller(9)); //-1
        System.out.println(nextSmaller(111)); //-1
        System.out.println(nextSmaller(135)); //-1
        System.out.println(nextSmaller(1027)); //-1
        System.out.println(nextSmaller(13966337089L)); // 13966330987
    }

    static long nextSmaller(long n) {
        char[] carr = String.valueOf(n).toCharArray();
        int len = carr.length, i;
        for (i = len - 1; i > 0; i--) {
            if (carr[i] < carr[i - 1]) break;
        }
        if (i == 0) return -1;
        else {
            int x = carr[i - 1], min = i;
            for (int j = i + 1; j < len; j++) {
                if (carr[j] < x && carr[j] > carr[min]) {
                    min = j;
                }
            }           
            char temp = carr[i-1];
            carr[i-1] = carr[min];
            carr[min] = temp;            
            String[] sarr = String.valueOf(carr).split("");            
            java.util.Arrays.sort(sarr, i, len, java.util.Collections.reverseOrder());
            long r = Long.valueOf(String.join("", sarr));
            return String.valueOf(r).length() == len ? r : -1;
        }
    }
}