longest common substring - brute force solution - O(n^3) - code after reviewing.

longestCommonSubstring3.cs

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

namespace longestCommonSubstring
{
    class Program
    {
        static void Main(string[] args)
        {
            string test = longestCommonSubstring("abc123def","hij123klmn"); 
        }

        /*
         * July 27, 2016 
         * 
         * Design:
         * Using the start position as a variable to do brute force search, 
         * Two strings - common substring 
         * iterate one string on start position, 
         * iterate second string on start position as well
         * 
         * Time complexity - O(n^4) -> O(n^3) 
         * instead of two variables - start position and end position - O(n^4)
         * 
         * Read C++ code fist to get the idea, 
         * 
         * and then, write C# code
         * 
         * Target: try to finish it in 20 minutes in first writing. 
         */
        public static string longestCommonSubstring(string s1, string s2)
        {
            if (s1 == null || s1.Length == 0 || s2 == null || s2.Length == 0)
                return string.Empty;

            int len1 = s1.Length;
            int len2 = s2.Length; 
            int maximum = 0;
            string res = string.Empty; 

            for(int i=0; i<len1; i++ )
                for (int j = 0; j < len2; j++)
                {
                    int start1 = i;
                    int start2 = j;
                    int cur = 0; 
                    while (start1 < len1 && start2 < len2)
                    {
                        char c1 = s1[start1];
                        char c2 = s2[start2];

                        if (c1 == c2)
                        {
                            start1++;
                            start2++;
                            cur++;
                        }
                        else
                        {
                            if (cur > maximum)
                            {                                 
                                maximum = cur; 

                                // get substring from s1 - start/ end position 
                                int end = start1 - 1; // s1 - start: i, end: start1 -1
                                res = s1.Substring(i, end - i + 1); 
                            }

                            break; 
                        }
                    }
                }

            return res; 
        }
    }
}

This does not work. try longestCommonSubstring("wonderfulhelloare", "helhelloare")