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:

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

public class Solution {
    /**
     * 也可以参考http://blog.csdn.net/u011095253/article/details/9158429的第二种方法
    **/
    public ArrayList<String> generateParenthesis(int n) {
        if (n<=0) {
            return new ArrayList<String>();
        }
        if (n==1) {
            ArrayList<String> ret = new ArrayList<String>();
            ret.add("()");
            return ret;
        }
        ArrayList<String> ret = generateParenthesis(n-1);
        HashSet<String> ret2 = new HashSet<String>();
        for (String temp : ret) {
            ret2.add("("+temp+")");
            ret2.add("()"+temp);
            ret2.add(temp+"()");
            //fix result (())(())
            for (int j=0;j<temp.length();j++) {
                if (temp.charAt(j) == '(') {
                    String fix = temp.substring(0, j+1) + "()" + temp.substring(j+1);
                    ret2.add(fix);
                }
            }
        }
        ret.clear();
        for (String s:ret2) {
            ret.add(s);
        }

        return ret;
    }
}