```
class Solution:
# @param haystack, a string
# @param needle, a string
# @return an integer
def strStr(self, haystack, needle):
ln = len(needle)
lh = len(haystack)
if ln > lh:
return -1
if ln == 0:#if needle is '' then return 0 for empty string is the substring of any string
return 0
for i in range(lh - ln + 1):#we only need consider the length of lh - ln
j = 0
while haystack[i + j] == needle[j]:
j += 1
if j == len(needle):
return i
return -1
```