using System;

public class Program

{

    public static void Main()

    {

        string[] s = {"flower", "flow", "flight"};

        Console.WriteLine(longestCommonPrefix(s));

    }

    public static string longestCommonPrefix(string[] s) {

        // assumptions: common preffix exists; strings given in lower case

        string commonPrefix = s[0];

​

​

        int length = 1;

        while(length < s[0].Length)

        {

            //Console.WriteLine(length);

            string pref = commonPrefix.Substring(0,length);

            foreach(string word in s)

            {

                if(word.Length>=length && word.Substring(0,length).Equals(pref))

                {

                    // not great I know (I will refactor later)

                }

                else

                {

                    return s[0].Substring(0, length-1);

                }

            }

            length++;

        }

        return s[0].Substring(0, length);

    }

}