using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<string> X = new List<string>{    "a",    "b",    "c",    "d",    "e",    "f",    "g", "h",  "i"};

        List<float>  Y = new List<float> { 0.991f, 1.471f, 3.819f, 0.003f, 2.291f, 2.887f, 2.887f,   0, 1.0f};

​

         var sorted = X

            .Select((value, index) => new { Index = index, Value = value })

            .OrderBy(o => Y[o.Index])

            .Select(o => o.Value);

        Console.WriteLine(string.Join(", ", sorted));

    }

}