“cd -” to toggle between the last two directories

ubuntu1004@ubuntu1004:~$ cd /tmp/tmp1/tmp2/tmp3
ubuntu1004@ubuntu1004:/tmp/tmp1/tmp2/tmp3$ cd /tmp/tmp10/tmp20/tmp30
ubuntu1004@ubuntu1004:/tmp/tmp10/tmp20/tmp30$ cd -
/tmp/tmp1/tmp2/tmp3
ubuntu1004@ubuntu1004:/tmp/tmp1/tmp2/tmp3$ cd -
/tmp/tmp10/tmp20/tmp30
ubuntu1004@ubuntu1004:/tmp/tmp10/tmp20/tmp30$ cd -
/tmp/tmp1/tmp2/tmp3
ubuntu1004@ubuntu1004:/tmp/tmp1/tmp2/tmp3$