NextLargestNumber.cs

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

namespace CodeADay
{
    class Program
    {
        static void Main(string[] args)
        {
            string numberGiven = "1234";
            char[] numberGivenAsArray = numberGiven.ToCharArray();

            List<string> permutationsList = new List<string>();

            for (int i = 0; i < numberGivenAsArray.Length; i++)
            {
                for (int j = 0; j < numberGivenAsArray.Length - 1; j++)
                {
                    // for each pass,
                    // swap the current index char with the char at the end of the array
                    char endChar = numberGivenAsArray[numberGivenAsArray.Length - 1];
                    char currentChar = numberGivenAsArray[j];

                    numberGivenAsArray[j] = endChar;
                    numberGivenAsArray[numberGivenAsArray.Length - 1] = currentChar;

                    // store the new swapped value.
                    permutationsList.Add(new string(numberGivenAsArray));
                }
            }

            // find the smallest distance between the given number and the permutations
            // ensuring the permutation is greater than the given number.
            int numberGivenAsInt = int.Parse(numberGiven);
            int tempDistanceToNumber = -1, tempClosestNumber = -1;

            foreach (var c in permutationsList)
            {
                int thisPermutation = int.Parse(c);
                int thisDistance = thisPermutation - numberGivenAsInt;

                if (tempDistanceToNumber == -1 || tempClosestNumber == -1)
                {
                    tempDistanceToNumber = thisDistance;
                    tempClosestNumber = thisPermutation;
                }
                else
                {
                    // ensure this distance is positive as that means
                    // this permutation is larger than the given number.
                    if (thisDistance > 0)
                    {
                        if (thisDistance < tempDistanceToNumber)
                        {
                            tempDistanceToNumber = thisDistance;
                            tempClosestNumber = thisPermutation;
                        }
                    }
                }
            }

            Console.WriteLine("Distance = {0}, Permutation = {1}", tempDistanceToNumber, tempClosestNumber);
            Console.ReadLine();
        }
    }
}