Problem

You are given an string $s$. You are also given two strings $t$ and $u$ which are both initially empty. You can perform either of the two operations in a turn:

• Append the first character of $s$ to the end of $t$.
• Append the last character of $t$ to the end of $u$.

You need to apply these operations such that all the end all the characters from $s$ are shifted to $u$, and the generated string $u$ is the lexicographically smallest possible. Return the lexicographically smallest string $u$ that can be generated.