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

namespace BK
{
    class Program
    {
        static void Main(string[] args)
        {

            Console.WriteLine(BF("abcdef", "abce"));
            Console.WriteLine(BF("abcdef
 
", "ab"));
            Console.WriteLine(BF("abcdef", "bc"));
            Console.WriteLine(BF("abcdef", "cd"));
            Console.WriteLine(BF("abcdef", "de"));
            Console.WriteLine(BF("abcdef", "ef"));
            Console.WriteLine(BF("abcdef", "ef3"));
            Console.ReadKey();
        }


        
 
private static int BF(string resStr, string pStr)
        {
            int result = -1;
            int resStrLength = resStr.Length;
            int pStrLength = pStr.Length;
            int i = 0, j = 0;
            while (i < resStrLength && j < pStrLength)
            {
                
 
if (resStr[i] == pStr[j])
                {
                    i++;
                    j++;
                }
                else
                {
                    i = i - j + 1;
                    j = 0;
                }
            }
            if (j >= pStrLength)
            {
                result = i - j;
            }

            return result;
        }
    }
}

運算結果：