空格替换%20

先遍历一遍字符串，找出空格个数，计算出新字符串的长度，从尾到头处理，减少字符移动

/**
 * Created by itworker365 on 5/12/2017.
 */
public class Main {
    public static void main(String[] args) {
        Main solution = new Main();
        StringBuffer sb = new StringBuffer();
        sb.append("this is test");
        String res = solution.replaceSpace(sb);
        System.out.println(res);
    }
    public String replaceSpace(StringBuffer str) {
        int spaceCnt = 0;
        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            if ((int)c == 32) {
                spaceCnt++;
            }
        }
        char[] temp = new char[str.length() + spaceCnt*2];
        System.out.println(spaceCnt);
        int tempIdx = temp.length - 1;
        for (int i = str.length() - 1; i >= 0; i--) {
            char c = str.charAt(i);
            if ((int)c == 32) {
                temp[tempIdx--] = '0';
                temp[tempIdx--] = '2';
                temp[tempIdx--] = '%';
            } else {
                temp[tempIdx--] = c;
            }
        }
        return new String(temp);
    }
}