I am definitely depressed. In the problem it says "input file", hence I think there is only one file and there are multi-tests in it. As a result I didn't test whether the next char is EOF and Crash again and again in test 1. No problem has been found. After submitting for 10+ times, I determine that I would rather WA Test 2 than Crash Test 1, so I truncate the final part of my program and sumbit, and this time it's AC. OMG, I am overwhelmed.