使用正则分割字符串最方便了

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
  public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    final String s = in.nextLine();
    String source = "I am a student";
    final String[] split = source.split("[^a-zA-Z]+");
    for (int i = split.length -1;  i >= 0; i--) {
      System.out.print(split[i]);
      if (i != 0) {
        System.out.print(" ");
      }
    }
  }
}