import java.util.*;class Main{public static void main(String[]g)throws Exception{byte[]u=new byte[32767];String x,y,c="";Map h=new HashMap();int d,n=0;g=new String(u,0,System.in.read(u)).split("\n");while(g.length>++n){x=g[n];y=new StringBuffer(x).reverse().toString();d=x.compareTo(y);if(d==0)c+=x;else if(h.get(x)!=null)h.put(x,d<0?x:y);else h.put(y,"");}x="";for(Object z:new TreeSet(h.values()))x+=z;System.out.print((x+c)+(new StringBuffer(x).reverse().toString()));}}

import java.util.*;class Main{public static void main(String[]g)throws Exception{byte[]u=new byte[32767];String x,y,c="";Map h=new HashMap();int d,n=0;g=new String(u,0,System.in.read(u)).split("\n");while(g.length>++n){y=new StringBuffer(x=g[n]).reverse().toString();d=x.compareTo(y);if(d==0)c+=x;else if(h.get(x)!=null)h.put(x,d<0?x:y);else h.put(y,"");}x="";for(Object z:new TreeSet(h.values()))x+=z;System.out.print(x+c+new StringBuffer(x).reverse().toString());}}

Update test5.java