Generate Parentheses的一个小问题

题目是：

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:

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

这段是可以通过的代码，但我如果把String换成StringBuffer就会超时，是为啥？

1. import java.util.ArrayList;
   
2. public class Solution {
   
3.     public ArrayList<String> generateParenthesis(int n) {
   
4.         ArrayList<String> result = new ArrayList<String>();
   
5.         if(n <= 0) return result;
   
6.         generateParen(result, n, n, "");
   
7.         return result;
   
8.     }
   
9.     private void generateParen(ArrayList<String> result, int leftRemain, int rightRemain, String sb){
   
10.         if(leftRemain < 0 || rightRemain < 0 || leftRemain > rightRemain) return;
    
11.         if(leftRemain == 0 && rightRemain == 0){
    
12.             result.add(sb);
    
13.         } else {
    
14.             generateParen(result, leftRemain - 1, rightRemain, sb+"(");
    
15.             generateParen(result, leftRemain, rightRemain - 1, sb+")");
    
16.         }
    
17.     }
    
18. }

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这是换成StringBuffer的代码

1. public ArrayList<String> generateParenthesis(int n) {
   
2.         ArrayList<String> result = new ArrayList<String>();
   
3.         if(n <= 0) return result;
   
4.         generateParen(result, n, n, new StringBuffer());
   
5.         return result;
   
6.     }
   
7.     private void generateParen(ArrayList<String> result, int leftRemain, int rightRemain, StringBuffer sb){
   
8.         if(leftRemain < 0 || rightRemain < 0 || leftRemain > rightRemain) return;
   
9.         if(leftRemain == 0 && rightRemain == 0){
   
10.             result.add(sb.toString());
    
11.         } else {
    
12.             generateParen(result, leftRemain - 1, rightRemain, sb.append("("));
    
13.             generateParen(result, leftRemain, rightRemain - 1, sb.append(")"));
    
14.         }
    
15.     }

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StringBuffer is mutable.
So everytime you do line 12 and when it comes back to execute line 13, 'sb' is changed with an additional '(', which may cause the problem. You may need to delete that '(' in your sb before you execute Line 13.
On the contrary String is immutatble. So sb in the first program would always stay the same.