Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

[
  "((()))",
  "(()())",
  "(())()",
  "()(())",
  "()()()"
]

class Solution {
    List<String> res = null;

	public void generate(int n, int left, int right, String s) {
		if (left + right == 2 * n) {
			res.add(s);
			return;
		}
		if (left < n) {
			generate(n, left + 1, right, s + "(");
			if (right < left) {
				generate(n, left, right + 1, s + ")");
			}
		} else {
			generate(n, left, right + 1, s + ")");
		}
	}

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