1 public class Solution {
 2     public ArrayList<String> generateParenthesis(int n) {
 3         // IMPORTANT: Please reset any member data you declared, as
 4         // the same Solution instance will be reused for each test case.
 5         ArrayList<String> result = new ArrayList<String>();
 6         StringBuilder builder = new StringBuilder();
 7         generate(result, builder, n, n);
 8         return result;
 9     }
10     private void generate(ArrayList<String> result, StringBuilder builder, int start, int end){
11         if(start == 0 && end == 0){
12             result.add(builder.toString());
13             return;
14         }
15         if(start > 0){
16             builder.append('(');
17             generate(result, builder, start-1, end);
18             builder.deleteCharAt(builder.length()-1);
19         }
20         if(end > start){
21             builder.append(')');
22             generate(result, builder, start, end-1);
23             builder.deleteCharAt(builder.length()-1);
24         }
25     }
26 }

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