Description:(PYTHON) Given a string consisting only of "1", "2", and "3". You can perform the following operations any number of times: exchange adjacent "1" and "2", or exchange adjacent "2" and "3". For example, for the string "121321", we can do the following: 121321 -> 211321; 121321 -> 112321; 121321 -> 112231; 121321 -> 112312; Note: Adjacent "1" and "3" cannot be exchanged. Output the smallest lexicographical string that can be obtained after performing the above operations any number of times (can be 0 times)