Method backtrack

java/0022-generate-parentheses.java:23–41

(int openN, int closedN, int n)

    }

    private void backtrack(int openN, int closedN, int n) {
        if (openN == closedN && closedN == n) {
            StringBuilder sb = new StringBuilder();
            for (Character c: stack) {
                sb.append(c);
            }
            res.add(sb.toString());
        }
        if (openN < n) {
            stack.push('(');
            backtrack(openN + 1, closedN, n);
            stack.pop();
        }
        if (closedN < openN) {
            stack.push(')');
            backtrack(openN, closedN + 1, n);
            stack.pop();
        }
    }
}