First non-repeating character in an array.

Count all array values in a hash map, then make a second pass in original order and return the first element whose frequency is one. The second pass matters: iterating the map does not preserve input order in general, and finding the smallest unique value is a different task. If the value range is tightly bounded, an array of counts can replace a map, but only after confirming bounds and negative-value handling. Decide what to return when no value is unique, such as null, -1, or an optional value, based on the interface. Test a singleton array, all duplicates, a unique value at index zero, multiple unique values, and negative or large keys. The algorithm is O(n) expected time and O(k) space for k distinct values. A nested scan is O(n squared) and should be reserved only for tiny constraints or explicit no-extra-space restrictions.