409047: GYM103427 F Encoded Strings I

Tommy has just invented an interesting string encoding algorithm, which is described below.

• For every string $$$S$$$, we may define a character-mapping function $$$F_S$$$, which maps every character occurring in $$$S$$$ to a lowercase letter, as below $$$$$$ F_S(c) = \operatorname{chr}(G(c, S)) $$$$$$ where $$$\operatorname{chr}(i)$$$ is the $$$(i+1)$$$-th lowercase Latin letter, and $$$G(c, S)$$$ is the number of different characters after the last occurrence of $$$c$$$ in $$$S$$$.
• To encode a string $$$S$$$ by Tommy's algorithm, replace every character $$$c$$$ in $$$S$$$ by $$$F_S(c)$$$ simultaneously.

For example, the encoded string of abc is cba, and the encoded string of cac is aba.

You are given a string of length $$$n$$$ and then encode all the $$$n$$$ nonempty prefixes. Your task is to find the encoded string that has the greatest lexicographical order among all the encoded strings.

The first line contains an integer $$$n$$$ $$$(1 \le n \le 1\,000)$$$.

The second line contains a string of length $$$n$$$, which consists of only the first $$$20$$$ lowercase letters, a to t.

Output the encoded string that has the greatest lexicographical order among all the encoded strings.

4
aacc

bbaa

3
aca

ba

In the first sample case, the four nonempty prefixes a, aa, aac and aacc will be encoded to a, aa, bba and bbaa respectively, where bbaa has the greatest lexicographical order.

In the second sample case, the three nonempty prefixes a, ac and aca will be encoded to a, ba and aba respectively, where ba has the greatest lexicographical order.