public static void main(String[] args) {
        // TODO Auto-generated method stub

        String s = "We are happy.";
        System.out.println(replaceSpace2(s));
    }

    //方法一：直接用String的replaceAll方法替换
    public static String replaceSpace(String s) {
        return s.replaceAll(" ", "%20");
    }
    //方法二：StringBuilder
    public static String replaceSpace1(String s) {
        StringBuilder sb = new StringBuilder();
        for(char ch:s.toCharArray()){
            if(ch==‘ ‘){
                sb.append("%20");
            }else{
                sb.append(ch);
            }
        }
        return sb.toString();
    }
    //方法三:字符数组
    public static String replaceSpace2(String s) {
        char[] result = new char[s.length()*3];
        int i=0;
        for(char ch:s.toCharArray()){
            if(ch==‘ ‘){
                result[i++]=‘%‘;
                result[i++]=‘2‘;
                result[i++]=‘0‘;
            }else{
                result[i++]=ch;
            }
        }
        return new String(result,0,i);
    }
}