题解

循环寻找每一个空格的位置，并替换

    public String replaceSpace(StringBuffer str) {
        int index_of_space = str.indexOf(" ");
        while (index_of_space != -1) {
            str.replace(index_of_space, index_of_space + 1, "%20");
            index_of_space = str.indexOf(" ");
        }
        return str.toString();
    }