Description
Dominoes are gaming pieces used in numerous tile games. Each doimno piece contains two marks. Each mark consists of a number of spots (possibly zero).
The number of spots depends on the set size. Each mark in a size N domino set can contain between 0 and N spots, inclusive. Two tiles are considered identical if their marks have the same number of spots, irregardles of reading order. For example tile with 2 and 8 spot marks is identical to the tile having 8 and 2 spot marks. A proper domino set contains no duplicate tiles. A complete set of size N contains all posible tiles with N or less spots and no duplicate tiles. For example, the complete set of size 2 contains 6 tiles:
Write a program that will determine the total number of spots on all tiles of a complete size N set.
Input
The first and only line of input contains a single integer, N (1 ≤ N ≤ 1000), the size of the complete set.
Output
The first and only line of output should contain a single integer, total number of spots in a complete size N set.
Sample Input
3
Sample Output
12
Source
这个题挺好玩的，我列出所有排列吧，第一行可以是0-n，对应下一行可以是0-n，这个第一行0-n的都要出现n+1次，上下相同的要除去，所以直接n*（n+1）（n+2）/2
#include<stdio.h> int main() { int n; scanf("%d",&n); printf("%d",n*(n+1)/2*(n+2)); }
原文：http://www.cnblogs.com/BobHuang/p/7822896.html