public String strStr(String haystack, String needle) {
        if(haystack == null) return null;
        if(needle == null || needle.length() == 0) return haystack;

        for(int i = 0; i <= haystack.length() - needle.length(); i++){
            int j = 0;
            for( ; j < needle.length(); j++){
                if(haystack.charAt(i+j) != needle.charAt(j)) break;
            }
            if(j == needle.length()){
                return haystack.substring(i);
            }
        }

        return null;
    }