lintcode: 生成括号

public class Solution {
    /**
     * @param n n pairs
     * @return All combinations of well-formed parentheses
     */
    public ArrayList<String> generateParenthesis(int n) {
        // Write your code here
        ArrayList<String> result  = new ArrayList<String>();
        if(n<=0)
            return result;
        int left = 0,right=0;
        String str="";
        generateParenthesis(n,left,right,str,result);
        return result;
    }
    public void generateParenthesis(int n,int left,int right,String str ,ArrayList<String> result){
        if(left == n){
            for(int i=0;i< n- right;i++)
                str+=")";
            result.add(str);
            return;
        }
        generateParenthesis(n, left+1, right,str+"(",result);  
        if(left>right){  
            generateParenthesis(n, left, right+1,str+")",result); 
        }  
    }
}