public String strStr(String haystack, String needle) {
        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;
	}