(CareerCup)Find next higher number with same digits

题目地址：http://www.careercup.com/question?id=4869907900530688

Find next higher number with same digits. 
Example 1 : if num = 25468, o/p = 25486 
Example 2 : if num = 21765, o/p = 25167 
Example 3 : If num = 54321, o/p = 54321 (cause it's not possible to gen a higher num than tiz with given digits ).

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace FindNextHigherNumberWithSameDigits
{
    class Program
    {
        static void Main(string[] args)
        {
            Do();
        }

        static void Do()
        {
            Console.WriteLine("plz input number: ");
            int input = Convert.ToInt32(Console.ReadLine());
            var arrayInput = input.ToString().ToCharArray();
            Find(ref arrayInput);
            Console.WriteLine("result : " + string.Join("", arrayInput));
            Do();
        }

        static void Find(ref char[] array)
        {
            int indexFirst = 0;
            int indexLast = array.Length - 1;
            bool hasChanged = false;
            for (int i = array.Length - 1; i > 0; i--)
            {
                //find the number larger than before one
                if (array[i] > array[i - 1])
                {
                    hasChanged = true;
                    indexLast = i;
                    indexFirst = i - 1;
                    //find the number smaller than the number we found 
                    //and larger than the previous number of the number we found
                    for (int a = array.Length - 1; a >= i; a--)
                    {

                        if (array[a] > array[i - 1] && array[a] < array[i] && array[a] <= array[indexLast])
                        {
                            indexLast = a;
                            indexFirst = i - 1;
                        }
                    }
                    break;
                }
            }
            //if the number is descending, return .
            if (!hasChanged) return;

            var temp = array[indexFirst];
            array[indexFirst] = array[indexLast];
            array[indexLast] = temp;
            //sort the number after previous position
            Array.Sort(array, indexFirst + 1, array.Length - indexFirst - 1);
        }
    }
}