LeeCode_URL化

class Solution {
    public String replaceSpaces(String S, int length) {
        StringBuilder sb = new StringBuilder();
        for (int i=0; i<length; i++){      // 以真实长度为界
            if(S.charAt(i)==' '){
                sb.append("%20");
            }else{
                sb.append(S.charAt(i));
            }
        }
        return sb.toString();
    }
}