[leetcode]Generate Parentheses

 1  public class Solution {
 2      List<String> result = new ArrayList<String>();
 3      public List<String> generateParenthesis(int n) {
 4          dfs(new StringBuilder(),0,0,n);
 5          return result;
 6      }
 7      private void dfs(StringBuilder sb,int k,int couple,int n){
 8          if(k == 0 && couple == n && sb.length() == 2 * n ){
 9              result.add(sb.toString());
10              return;
11          }
12          if( k < 0 || sb.length() > 2 * n) return;//剪枝
13          char[] c = {‘(‘,‘)‘};
14          for(int i = 0; i < 2; i++){
15             k = (i == 0 )? k + 1 : k-1;
16             if(k > 0 && i == 1) {//配对的前提是k>0，即存在未配对的左括号
17                  couple++;
18                  k--;
19              }
20 　　　　     if(k < 0){//如果前面木有左括号，则只能匹配右括号
21                  k++;
22                  continue;
23              }
24              dfs(sb.append(c[i]), k,couple, n);
25              sb.deleteCharAt(sb.length() - 1);
26          }
27      }
28  }