Generate Parentheses

public class Solution {
    public ArrayList<String> generateParenthesis(int n) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        ArrayList<String> result = new ArrayList<String>();
        if(n == 0) return result;
        char[] temp = new char[n*2];
        int left = 0;
        int right = 0;
        int count = 0;
        helpler(left, right, count, n, temp, result);
        return result;
    }
    private void helpler(int left, int right, int count, int n, char[] temp, ArrayList<String> result)
    {
        if(left>n || right>left) return;  //invalid state
        if((left+right) == (n*2)) result.add(String.copyValueOf(temp));
        else
        {
            if(left<n){                         //add "(" if any left
                temp[count] = '(';
                helpler(left+1, right, count+1, n, temp, result);
            }
            if(right<left){                      //add ")" is valid, only when # of "(" is larger than the # of ")"
                temp[count] = ')';
                helpler(left, right+1, count+1, n, temp, result);
            }
        }
    }
}