正整数 n 代表生成括号的对数，请设计一个函数，用于能够生成所有可能的并且 有效的 括号组合。
示例 1：
输入：n = 3
输出：["((()))","(()())","(())()","()(())","()()()"]示例 2：
输入：n = 1
输出：["()"]提示：
1 <= n <= 8
//leetcode submit region begin(Prohibit modification and deletion)
class Solution {
public List<String> generateParenthesis(int n) {
List<String> result = new ArrayList<>();
doGenerate(result, "", 0, 0, n);
return result;
}
private void doGenerate(List<String> result, String curRes, int leftNum, int rightNum, int n) {
if (leftNum == n && rightNum == n) {
result.add(curRes);
return;
}
if (leftNum < n) {
doGenerate(result, curRes + "(", leftNum + 1, rightNum, n);
}
if (rightNum < n && rightNum < leftNum) {
doGenerate(result, curRes + ")", leftNum, rightNum + 1, n);
}
}
}
//leetcode submit region end(Prohibit modification and deletion)
