Top k error codes

A log pipeline collects error codes from a web service in the order they occurred. Given that ordered list and an integer k, return the k codes that occurred most often, highest count first. When two codes have equal counts, the one whose first occurrence came earlier in the log wins the earlier spot. Assume 1 <= k <= number of distinct codes. Example: codes = ["504", "500", "504", "500", "404"], k = 2 gives ["504", "500"], both occur twice, and "504" appeared first.