int binarySearch(String[] a, String key){
            int low = 0; //開始位置
            int high = a.length -1; //結束位置


            while (low <= high) {
                int mid = (low + high) >>> 1; //相當于mid = (low + high)/2
                String midVal = a[mid]; //取中間的值
                int cmp = midVal.compareTo(key); //中間值和要找的關鍵詞比較


                if (cmp < 0)
                    low = mid + 1;
                else if (cmp > 0)
                    high = mid -1;
                else
                    return mid; // 查找成功，返回找到的位置
            }
            return -(low + 1);  // 沒找到，返回負值
        }