Suppose you are given the task of printing all characters in a string string that occur exactly once. For example, given the string

"reader", you would print a and d because $r$ and $e$ are repeated.

That is not easy, so let's solve a simpler problem first. Does the first character occur exactly once?

Start with the algorithm from this section. You need to modify the algorithm so that you look for the first character, and you need

to look for it in the rest of the string.

Rearrange the following lines of code to produce a solution. Not all lines are useful.