Library Queue Management

You manage a small library's book lending system. Books are lent out and returned in a first-come, first-served order. A queue is implemented for this process. Your task is to simulate the lending and returning of books based on a series of commands. Each command can be either 'lend <book_id>' to lend a book or 'return <book_id>' to return a book. Output the order in which books are lent out in a single line when all commands are processed. Input Format: - The first line contains an integer n (1 <= n <= 1000), the number of commands. - The next n lines contain one command each in the format described above. Output Format: - A single line of space-separated book IDs representing the order of lent books. Example: Input: 5 lend 101 lend 102 return 101 lend 103 return 102 Output: 101 102 103