Find First Non-Repeating Character

You're given a string of random alphanumerical characters with a length between 0 and 1000.

Write a method to return the first character in the string that does not repeat itself later on.

So if we had a string input of "asdfsdafdasfjdfsafnnunl'", we can see there are multiple letters that are repeated.

Executing firstNonRepeat('asdfsdafdasfjdfsafnnunl') should return 'j' since it's the first letter where there is no duplicate.

Constraints

• The given string can be empty
• The string will only contain lowercase/uppercase alphabets and numerals
• Expected time complexity : O(n)
• Expected space complexity : O(A) where A is the number of ASCII characters

```js
function firstNonRepeat(str) {
  // fill this in
  return str;
};
```