> 我就会用穷举法。。然后n=1000显然超时了呀。。。
> 
> There Numbers
> Submit: 10   Accepted:1
> Time Limit: 1000MS  Memory Limit: 65536K
> Description
> Given a positive integer n.Choose 3 distinguishing numbers a1,a2,a3 from 1 to n(1<=a1,a2,a3<=n),find the number of ways that satisfying the equation (a1+a2+a3)%3=0.
> 
> Input
> A positive integer n(0 < n < 1000).
> 
> 
> Output
> The number of ways that satisfying the equation.
> 
> 
> Sample Input
> 
> 3
> 
> 
> Sample Output
> 
> 1
> 
> 
> Source