Generate Parentheses - JAVA

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:

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

java code : 采用递归树的思想，当左括号数大于右括号数时可以加左或者右括号，否则只能加左括号，当左括号数达到n时，剩下全部

加右括号。

public class Solution {  
    public ArrayList generateParenthesis(int n) {  
        // Note: The Solution object is instantiated only once and is reused by each test case.  
        ArrayList




 res = new ArrayList();  
        generate(res, "", 0, 0, n);  
        return res;  
    }  
    public void generate(ArrayList res, String tmp, int lhs, int rhs, int n)  
    {  
        if(lhs == n)  
        {  
            for(int i = 0; i < n - rhs; i++)  
            {  
                tmp += ")";  
            }  
            res.add(tmp);  
            return ;  
        }  
        generate(res, tmp + "(", lhs + 1, rhs, n);  
        if(lhs > rhs)  
            generate(res, tmp + ")", lhs, rhs + 1, n);  
    }  
}