题解 | 牛牛学数列4
牛牛学数列4
https://www.nowcoder.com/practice/586b114f52034688879342ba45779081
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
int n=in.nextInt();
/*最开始是long sum=0;
for(int i=1;i<=n;i++)
sum+=i*(n-i+1)//很容易看出有n个1，n-1个2……然后又有1个n，2个n-1与前面对称于是又优化*/
long sum=n*(n+1)*(n+2)/6;
System.out.print(sum);
}
}
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