Java 解法一：
public class Solution {
public List<String> generateParenthesis(
int n) {
List<String> res =
new ArrayList<String>
();
helper(n, n, ""
, res);
return res;
}
void helper(
int left,
int right, String out, List<String>
res) {
if (left < 0 || right < 0 || left > right)
return;
if (left == 0 && right == 0
) {
res.add(out);
return;
}
helper(left - 1, right, out + "("
, res);
helper(left, right - 1, out + ")"
, res);
}
}
