Find the First Unique Log

You are given a list of log entries in the order they were produced. Each log entry can be treated as a string. Return the first log entry that appears exactly once in the entire list. If no such log entry exists, return `null` or an equivalent empty result. Example: Input: `["login", "click", "logout", "login", "purchase", "click"]` Output: `"logout"` Follow-up: How would your approach change if the input contains about 10 million log entries? Discuss time complexity, memory usage, and any practical considerations for large-scale processing.