Generate Parentheses

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

"((()))", "(()())", "(())()", "()(())", "()()()"

 1 public class Solution {
 2     ArrayList<String> result = null;
 3     public ArrayList<String> generateParenthesis(int n) {
 4         // Note: The Solution object is instantiated only once and is reused by each test case.
 5         StringBuffer st = new StringBuffer();
 6         result = new ArrayList<String>();
 7         if(n == 0) return result;
 8         getRow(st, n , n, 0);
 9         return result;
10     }
11     public void getRow(StringBuffer st, int n, int m, int num){
12         if(n == 0 && m == 0){
13             result.add(st.toString());
14         }
15         if(n > 0){
16             st.append('(');
17             num ++;
18             getRow(st, n - 1, m, num);
19             num --;
20             st.deleteCharAt(st.length() - 1);
21         }
22         if(m > 0 && num > 0){
23             st.append(')');
24             num --;
25             getRow(st, n, m - 1, num);
26             num ++;
27             st.deleteCharAt(st.length() - 1);
28         }
29     }
30 }

第三遍：

 1 public class Solution {
 2     public List<String> generateParenthesis(int n) {
 3         List<String> result = new ArrayList<String>();
 4         generate(result, 0, n, n, new StringBuilder());
 5         return result;
 6     }
 7     
 8     public void generate(List<String> result,int cur,int left,int right, StringBuilder sb){
 9         if(left == 0 && right == 0) result.add(sb.toString());
10         if(left > 0){
11             StringBuilder sbs = new StringBuilder(sb);
12             sbs.append('(');
13             generate(result, cur + 1, left - 1, right, sbs);
14         }
15         if(right > 0 && cur > 0){
16             StringBuilder sbs = new StringBuilder(sb);
17             sbs.append(')');
18             generate(result, cur - 1, left, right - 1, sbs);
19         }
20     }
21 }