Hash tables are very common data structures used for coding problems presented in competitive programming/interviews.

Hash tables take key value pairs so that you can lookup a key and get the value. However, I often find myself needing the `O(1)` lookup of a key and not really caring about the value.

**For example:**

If I need to know if some `strings` have been used previously, I might plug them into a hash table with `key: string`, `value: bool` where the value of the bool is always true.

What are the down sides of doing something like this? Are there other data structures that give O(1) lookup that don't need a key value pair?