Maximize the Remaining String

You are given a string $$$s$$$, consisting of lowercase Latin letters. While there is at least one character in the string $$$s$$$ that is repeated at least twice, you perform the following operation:

• you choose the index $$$i$$$ ($$$1 \le i \le |s|$$$) such that the character at position $$$i$$$ occurs at least two times in the string $$$s$$$, and delete the character at position $$$i$$$, that is, replace $$$s$$$ with $$$s_1 s_2 \ldots s_{i-1} s_{i+1} s_{i+2} \ldots s_n$$$.

For example, if $$$s=$$$"codeforces", then you can apply the following sequence of operations:

• $$$i=6 \Rightarrow s=$$$"codefrces";
• $$$i=1 \Rightarrow s=$$$"odefrces";
• $$$i=7 \Rightarrow s=$$$"odefrcs";

Given a given string $$$s$$$, find the lexicographically maximum string that can be obtained after applying a certain sequence of operations after which all characters in the string become unique.

A string $$$a$$$ of length $$$n$$$ is lexicographically less than a string $$$b$$$ of length $$$m$$$, if:

• there is an index $$$i$$$ ($$$1 \le i \le \min(n, m)$$$) such that the first $$$i-1$$$ characters of the strings $$$a$$$ and $$$b$$$ are the same, and the $$$i$$$-th character of the string $$$a$$$ is less than $$$i$$$-th character of string $$$b$$$;
• or the first $$$\min(n, m)$$$ characters in the strings $$$a$$$ and $$$b$$$ are the same and $$$n < m$$$.

For example, the string $$$a=$$$"aezakmi" is lexicographically less than the string $$$b=$$$"aezus".