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

public class Test
{
	public static void Main()
	{
        var input = "one two four";
        var replace = new Dictionary<string, string>()
        {
            { "one", "two" },
            { "two", "three" },
        };
    	
    	Console.WriteLine(ReplaceOnce(input, replace));
	}
    
    static string ReplaceOnce(string input, IDictionary<string, string> replacements)
    {
        var processedCharCount = 0;
        var sb = new StringBuilder();
    	while (processedCharCount < input.Length) {
    		var replacement = replacements.Select(r => new { Term = r.Key, Index = input.IndexOf(r.Key, processedCharCount)})
    					                  .Where(p => p.Index != -1)
    					                  .OrderBy(p => p.Index).ThenByDescending(p => p.Term.Length)
    									  .FirstOrDefault();
    									  
    		if (replacement == null)
    		{
    			break;
    		}
    
    		sb.Append(input, processedCharCount, replacement.Index - processedCharCount);
    		sb.Append(replacements[replacement.Term]);
    		processedCharCount = replacement.Index + replacement.Term.Length;
    	}
    	
    	sb.Append(input.Substring(processedCharCount));
    	return sb.ToString();
    }
}