查找字符串b在a中的起始位置，如果b不为a的子串，则返回-1

class Solution(object):
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        if not needle:
            return 0
        
        n_len = len(needle)
        h_len = len(haystack)
        
        if n_len > h_len:
            return -1
        elif n_len == h_len:
            if haystack == needle:
                return 0
            return -1
        for i in range(h_len-n_len+1):
            if haystack[i:i+n_len] == needle:
                return i
        return -1