1.     public List<String> generateParenthesis(int n) {
    2.         List<String> res = new LinkedList<>();
    3.         generate2(0, 2 * n, "", res);
    4.         return res;
    5.     }
    7.     void generate2(int level, int n, String s,List<String> res) {
    8.         if (level >= n) {
    9.             Stack<Character> stack = new Stack();
    10.             for (int i = 0; i < s.length(); i++) {
    11.                 if ('(' == s.charAt(i)) {
    12.                     stack.push(')');
    13.                 } else if (stack.isEmpty() || stack.pop() != s.charAt(i)) {
    14.                     return;
    15.                 }
    16.             }
    17.             if (stack.isEmpty()) {
    18.                 res.add(s);
    19.             }
    20.             return;
    21.         }
    22.         generate2(level + 1, n, s + "(",res);
    23.         generate2(level + 1, n, s + ")",res);
    24.     }