找出缺失的数
序列找数
http://www.nowcoder.com/questionTerminal/a7d1856a72404ea69fdfb5786d65539c
等差数列求和，减去已输入的数字的和，差即为未出现的：
import java.util.*;
public class Main {
public static void main(String[] args) throws Exception{
Scanner sc = new Scanner(System.in);
int n = sc.nextInt();
int sum = 0;
while(sc.hasNext()) sum += sc.nextInt();
System.out.println((1 + n)*n/2 - sum);
}
}