You are given a string s.

In one operation, you simultaneously remove the first occurrence of every distinct character present in s. This process is repeated until the string becomes empty.

Return the string s immediately before the last operation that makes it empty.

Constraints:

• $1 \leq$ s.length $\leq 5 imes 10^5$

• s consists only of lowercase English letters.

Now, let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Implement your solution in the following coding playground.