Missing Digit

Find the missing number and make the sequence complete again!

In Missing Digit, every string of numbers hides a logical password. The system will show a seemingly simple but hidden sequence-it may be arithmetic increment, proportional scaling, Fibonacci cyclotron, symmetrical structure or clever repetition. But one of them has been "?" Replace and wait for you to restore it with insight.
Your task is not to remember, but to reason: observe known numbers, identify patterns, calculate or infer the only correct answer, and enter only the missing number. No guessing, no trial and error-there is only one real solution.
Missing Digit is not only a small game, but also an efficient activator of daily brain power. Suitable for students to consolidate the understanding of sequence, programmers to train pattern recognition, or any player who loves logical challenges to enjoy the pleasure of "an epiphany" in the fragmentation time.