CodeWars/4kyu/next_bigger_with_same_digits/solution.cs

using System;
using System.Collections.Generic;
using System.Linq;

public class Kata {
    public static List<long> ExtractDigits(long n) {
        var result = new List<long>();

        while (n > 0) {
            result.Add(n % 10);
            n /= 10;
        }

        result.Reverse();
        return result;
    }

    public static bool NextPermutation(List<long> digits) {
//         foreach (var digit in digits) Console.Write($"{digit} ");
//         Console.WriteLine();

        var k = -1;

        for (var i = 0; i < digits.Count - 1; i++) {
            if (digits[i] < digits[i + 1])
                k = i;
        }
        if (k == -1) return false;

        var l = k;
        for (var i = 0; i < digits.Count; i++)
            if (digits[k] < digits[i]) l = i;
        if (l == k) return false;

        var tmp = digits[k];
        digits[k] = digits[l];
        digits[l] = tmp;

//         foreach (var digit in digits) Console.Write($"{digit} ");
//         Console.WriteLine();

        digits.Reverse(k + 1, digits.Count - k - 1);

//         foreach (var digit in digits) Console.Write($"{digit} ");
//         Console.WriteLine();

        return true;
    }

    public static long NextBiggerNumber(long n) {
      if (n < 10) return -1;

      var digits = ExtractDigits(n);
      if (!NextPermutation(digits)) {
          return -1;
      } else {
          var result = 0l;
          foreach (var digit in digits) {
              result *= 10;
              result += digit;
          }
          return result;
      }
    }
}