問題5 http://www.softantenna.com/wp/software/5-programming-problems/

gistfile1.cs

static void Main(string[] args)
{
    var sources = "123456789";
    var comm = " +-";

    var insertPoint = sources.Length - 1;
    var commCount = comm.Length;

    const int value = 100;

    System.Data.DataTable dt = new System.Data.DataTable();

    foreach (var v in Enumerable.Range(0, (int)Math.Pow(commCount, insertPoint)))
    {
        var formula = sources;
        foreach (var n in Enumerable.Range(0, insertPoint).Reverse())
        {
            var index = (v / (int)Math.Pow(commCount, n)) % commCount;
            formula = (index == 0) ? formula : formula.Insert(n + 1, new string(new[] { comm.ElementAt(index) }));
        }

        var result = (int)dt.Compute(formula, "");
        if (result == value)
            Console.WriteLine(formula + " = " + result);
    }
}