提交程式碼（遞迴）：

class Solution {
   List<String> generateParenthesis(int n) {
    	List<String> list=new ArrayList<>();
		append(n,n,"",list);
		return list;
}
  void append(int left,int right,String cur,List<String> list) {
    	if(left<0||right<0||left>right)	return;
    	if
 
(left==0&&right==0) {
    		list.add(cur);
    		return;
    	}
    	
    	append(left-1,right,cur+"(",list);
    	append(left,right-1,cur+")",list);
    }
}