import java.util.*;
public class SubAndSup
{
    public static String setString(String s)
      {
        StringBuffer str=new StringBuffer(s);

        StringBuffer strSup1=new StringBuffer("^{");
        StringBuffer strSup2=new StringBuffer("}");
        StringBuffer strSub1=new StringBuffer("_{");
        StringBuffer strSub2=new StringBuffer("}");

/*---------------------------第一部分-------------------------------------*/
        int length1=str.length();
        for(int i=0;i<length1;i++) //插入<sub>
           {
             if(Character.isLetter(str.charAt(i)) & Character.isDigit(str.charAt(i+1)))
               {
                 str.insert(i+1,strSub1);
                 str.insert(i+7,strSub2);
               }
             length1=str.length()-1;
           }

/*-----------------------------第二部分-----------------------------------*/       
        int length2=str.length();
        for(int i=0;i<length2;i++) //插入<sup>
           {
             if((Character.isDigit(str.charAt(i)) & str.charAt(i+1)=='+') || (Character.isDigit(str.charAt(i)) & str.charAt(i+1)=='-'))
                {
                   str.insert(i,strSup1);
                   str.insert(i+6,strSup2);
                }
             length2=str.length()-1;
           }

/*------------------------------第三部分---------------------------------*/
        int length3=str.length();
        for(int i=0;i<length3;i++)
           {
            int number1=str.lastIndexOf("</sup>+");
            if(number1<0 || number1>length3)
               {
                break;
               }
            else
               {
                 str.replace(number1,number1+7,"+</sup>");
               }
           }
        for(int i=0;i<length3;i++)
           {
             int number2=str.lastIndexOf("</sup>-");
             if(number2<0 || number2>length3)
               {
                 break;
               }
             else
               {
                 str.replace(number2,number2+7,"-</sup>");
               }
           }  
        String strString=str.toString();
        return strString;
      }
}

/*--------------------
public class Example009
{
    public static void main(String args[])
      {
        SubAndSup as=new SubAndSup();
        String ss1="H2O";
        String ss2=as.setString(ss1);
        System.out.print(ss2);
      }
}

打印输出：H₂O