How to Use “cd -” to toggle between the last two directories

You can toggle between the last two current directories using cd - as shown
below.
# cd /tmp/very/long/directory/structure/that/is/too/deep
# cd /tmp/subdir1/subdir2/subdir3
# cd -
# pwd
/tmp/very/long/directory/structure/that/is/too/deep
# cd -
# pwd
/tmp/subdir1/subdir2/subdir3
# cd -
# pwd
/tmp/very/long/directory/structure/that/is/too/deep