class Solution {
    public String replaceSpace(String s) {
        String  res  = "";
        for(int i = 0 ; i< s.length() ;i++){
            if(s.charAt(i)==' ')res+="%20";
            else res+=s.charAt(i);
        }
        return res;
  
    }
}

更高效的應該是使用了StringBuilder

class Solution {
    public String replaceSpace(String s) {
        StringBuilder sb = new StringBuilder();
        for(int i = 0 ; i< s.length() ;i++){
            if(s.charAt(i)==' ')sb.append("%20");
            else sb.append(s.charAt(i));
        }
        return sb.toString();
  
    }
}