Unconventional approach (all not pure bash):

• if all elements in an array are just one characters (like in the question) you can use rev:

   echo "${array[@]}" | rev
  

• otherwise:

   printf '%s\n' "${array[@]}" | tac | tr '\n' ' '; echo
  

• and if you can use zsh:

   echo ${(Oa)array}
  

Unconventional approach (all not pure bash):

• if all elements in an array are just one characters (like in the question) you can use rev:

   echo "${array[@]}" | rev
  

• otherwise if none of the array elements contain a new line:

   printf '%s\n' "${array[@]}" | tac | tr '\n' ' '; echo
  

• and if you can use zsh:

   echo ${(Oa)array}