Method generateParenthesis

java/0022-generate-parentheses.java:18–21

(int n)

    List<String> res = new ArrayList<>();

    public List<String> generateParenthesis(int n) {
        backtrack(0, 0, n);
        return res;
    }

    private void backtrack(int openN, int closedN, int n) {
        if (openN == closedN && closedN == n) {