括號回溯算法

經典回溯算法， 輸入數字，輸出對應括號對

    public static void main(String[] args) {
        int max=3;
        ArrayList result = new ArrayList();
        backtrack(result, "", 0, 0 , max);
        for (int i = 0; i < result.size(); i++) {
            System.out.println(result.get(i));
        }
    }
    public static void backtrack(ArrayList result , String str , int left , int right , int max){
        if (str.length() >= max*2 ){
            result.add(str);
            return;
        }
        if (left < max ){
            backtrack(result, str+"(", left+1, right, max);
        }
        if (right