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 }