Problem:
Score : 400 points

Problem Statement

A positive integer X is said to be a lunlun number if and only if the following condition is satisfied:

In the base ten representation of X (without leading zeros), for every pair of two adjacent digits, the absolute difference of those digits is at most 1.

For example, 1234, 1, and 334 are lunlun numbers, while none of 31415, 119, or 13579 is.

You are given a positive integer K. Find the K-th smallest lunlun number.

Constraints

1 \leq K \leq 10^5

All values in input are integers.

Input

Input is given from Standard Input in the following format:

K

Output

Print the answer.

Sample Input 1

15

Sample Output 1

23

We will list the 15 smallest lunlun numbers in ascending order:

1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
21,
22,
23.

Thus, the answer is 23.

Sample Input 2

1

Sample Output 2

1

Sample Input 3

13

Sample Output 3

21

Sample Input 4

100000

Sample Output 4

3234566667

Note that the answer may not fit into the 32-bit signed integer type.

Solution in C# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using static System.Console;
using static System.Math;

class program
{
    static public int mod = 1000000007;
    static void Main()
    {
        var k = rint();
        long ans = 0;
        var lunlun = new long[11];
        var isdig = new bool[11];
        for(int i = 0; i < k; i++)
        {
            lunlun[0]++;

            for(int j = 0; j < 10; j++)
            {
                if ((lunlun[j] > lunlun[j+1] + 1 && isdig[j+1]) || lunlun[j] > 9)
                {
                    lunlun[j + 1]++;
                    isdig[j + 1] = true;
                    if(lunlun[j + 1] < 10)
                        lunlun[j] = lunlun[j + 1] - 1;
                    else
                        lunlun[j] = 0;

                }
            }
        }
        long dig = 1;
        for(int j = 0; j < 10; j++)
        {
          //WriteLine(lunlun[j]);
            ans += dig * lunlun[j];
            dig *= 10;
        }
        Write(ans);
    }

    static int pow(int a, int b) { string binary = Convert.ToString(b, 2); int bileng = binary.Length; long a_power = a; long value = 1; for (int i = bileng - 1; i >= 0; i--) { if (binary[i] == '1') { value = value * a_power % mod; } a_power = a_power * a_power % mod; } return (int)value; }
    static string rstr() { return ReadLine(); }
    static int rint() { return int.Parse(ReadLine()); }
    static long rlong() { return long.Parse(ReadLine()); }
    static string[] stra() { return ReadLine().Split(' '); }
    static char[] chara() { string[] a = stra(); string b = ""; for (int i = 0; i < a.Length; i++) { b += a[i]; } return b.ToCharArray(); }
    static int[] inta() { string[] read_str_array = ReadLine().Split(' '); int[] return_int_array = new int[read_str_array.Length]; for (int countup_i = 0; countup_i < read_str_array.Length; countup_i++) { return_int_array[countup_i] = int.Parse(read_str_array[countup_i]); } return return_int_array; }
    static long[] longa() { string[] read_str_array = ReadLine().Split(' '); long[] return_long_array = new long[read_str_array.Length]; for (long countup_i = 0; countup_i < read_str_array.Length; countup_i++) { return_long_array[countup_i] = long.Parse(read_str_array[countup_i]); } return return_long_array; }
    static double[] doublea() { string[] read_str_array = ReadLine().Split(' '); double[] return_double_array = new double[read_str_array.Length]; for (long countup_i = 0; countup_i < read_str_array.Length; countup_i++) { return_double_array[countup_i] = double.Parse(read_str_array[countup_i]); } return return_double_array; }
    static long GCD(long a, long b) { if (a < b) { long temp = a; a = b; b = temp; } if (a % b == 0) { return b; } else { long temp = b; b = a % b; a = temp; return GCD(a, b); } }
    static long LCM(long a, long b) { return a * b / GCD(a, b); }
    static void WriteArray2(int[,] a, int b, int c) { for (int i = 0; i < b; i++) { for (int j = 0; j < c; j++) { if (j != 0) Write(" "); Write(a[i, j]); } WriteLine(); } }
    static void WriteArray(int[] a, int b) { for (int i = 0; i < b; i++) { Write(" "); Write(a[i]); } WriteLine(); }
}