28. 实现strStr()

public class Solution {
    public int StrStr(string haystack, string needle) {
       int fstrIndex = 0;
            int index = 0;
            if (needle == "")
            {
                return 0;
            }
            if (haystack.Length < needle.Length)
            {
                return -1;
            }
            if (haystack.Length == needle.Length)
            {
                if (haystack != needle)
                {
                    return -1;
                }
            }
            for (int i = 0; i < haystack.Length; i++)
            {
                if (haystack[i] == needle[fstrIndex])
                {
                    index = i;
                    for (int j = 0; j <needle.Length && index<haystack.Length; j++)
                    {
                        if (haystack[index] != needle[j])
                        {
                            break;
                        }
                        else if (haystack[index]==needle[j] && j == needle.Length -1)
                        {
                            return i;
                        }
                        index++;
                    }
                }
            }
            return -1;
    }
}