Input ["LUPrefix", "upload", "longest", "upload", "longest", "upload", "longest"] [[4], [3], [], [1], [], [2], []] Output [null, null, 0, null, 1, null, 3] Explanation LUPrefix server = new LUPrefix(4); // Initialize a stream of 4 videos. server.upload(3); // Upload video 3. server.longest(); // Since video 1 has not been uploaded yet, there is no prefix. // So, we return 0. server.upload(1); // Upload video 1. server.longest(); // The prefix [1] is the longest uploaded prefix, so we return 1. server.upload(2); // Upload video 2. server.longest(); // The prefix [1,2,3] is the longest uploaded prefix, so we return 3.