Run time error in given code. what do i need to change

#1

public static String dup(String str)
{
String ans="";

    if(str.substring(0)=="")
    {
        return ans ;
    }

    char c1=str.charAt(0);
    String ros=str.substring(1);

    ans=dup(ros);
    if(c1==ans.charAt(0))
    {
        return c1+"*"+ans;
    }
    else
    {
        return c1+ans;
    }
}
#2

error is becoz of u have ans length==0
and u trying to access its first char

hope this will help u
pls rate my work

#3

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