小米第一题，应该AC了吧，本地测试全通过，

import java.util.Scanner;
public class Main {
public static void main(String[] args) throws IOException {
int w = 0;
String s = "";
String b = "";
while (true) {
Scanner in = new Scanner(System.in);
String readLine = in.nextLine();
char[] a = readLine.toCharArray();
for (int j = 0; j < readLine.length(); j++) {
char i = a[j];
if ((i >= 'A' && i <= 'Z')) {
b = String.valueOf(i);

if (w == 3) {
s = s + b.toUpperCase();
w = 1;
} else {
s = s + "_" + b.toUpperCase();
w = 1;
}
} else if ((i >= 'a' && i <= 'z')) {
b = String.valueOf(i);
if (w == 1) {
s = s + b.toUpperCase();
} else {
s = s + "_" + b.toUpperCase();
w = 1;
}
} else if ((i >= '0' && i <= '9')) {
b = String.valueOf(i);
if (w == 2) {
s = s + b.toUpperCase();
} else {
s = s + "_" + b.toUpperCase();
w = 2;
}
} else if (i == '.') {
s = s + "_";
w = 3;
}

}
System.out.println(s + "_");
s = "";
}
}

}
#小米#