class Solution {
public String replaceSpace(String s) {
char[] ch = s.toCharArray();
//String res = "";
StringBuffer res = new StringBuffer();//用stringbuffer比用string效率更高
for(int i = 0 ; i < s.length() ; i++){
if(ch[i] == ' '){
res.append("%20");
}
else{
res.append(ch[i]);
}
}
return res.toString();
}
}