28 implement strstr()

class (object):
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        if needle == '':
            return 0
        if haystack == '':
            return -1
        result = -1
        length = len(needle)
        for i in range(len(haystack)):
            if i+length <= len(haystack):
                string = haystack[i:i+length]
                if needle == string:
                    return i
        return -1

class (object):
def strStr(self, haystack, needle):
    for i in range(len(haystack) - len(needle)+1):
        if haystack[i:i+len(needle)] == needle:
            return i
    return -1