Suppose there are n students and each student is numbered a different number in the range of [1, n]. For some class, the teacher realizes that there is a student missing. The teacher does not want to do a roll call and what the teacher has are the numbers of the students in class. Your job is to help the teacher finding out which student (number) is missing. (You can think that all the numbers are stored in an array of size n-1)

假設有n個學生，並且每個學生在[1, n]的範圍內給定不同的編號。對於某些課程，老師發現到有一個學生不見了。 老師不想點名，老師只有班上學生的數量。你的工作是幫助老師找出不見的學生（編號）。（您可以認為所有數字都存儲在大小為n-1的陣列中）