Implement strStr().
Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
func strStr(haystack string, needle string) int {
if len(haystack)==0&&len(needle)==0{
return 0
}
l:=len(needle)
for i:=0;i<len(haystack);i++{
if l+i<=len(haystack)&&haystack[i:i+l]==needle{
return i
}
}
return -1
}