public List<String> generateParenthesis(int n) {
    List<String> list = new ArrayList<String>();
    backtrack(list, "", 0, 0, n);
    return list;
}

public void backtrack(List<String> list, String str, int open, int close, int max){

    if(str.length() == max*2){
        list.add(str);
        return;
    }

    if(open < max)
        backtrack(list, str+"(", open+1, close, max);
    if(close < open)
        backtrack(list, str+")", open, close+1, max);
}