28. Implement strStr()

class Solution:
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        if not needle:
            return 0

        m = len(haystack)
        n = len(needle)

        if m < n:
            return -1

        for i in range(0, m-n+1):
            k = i
            j = 0
            while j < n:
                if haystack[k] == needle[j]:
                    j = j + 1
                    k = k + 1
                else:
                    break

            if j == n:
                return i

        return -1