HDU1396：Counting Triangles

Problem Description

Given an equilateral triangle with n the length of its side, program to count how many triangles in it.

 

Input

The length n (n <= 500) of the equilateral triangle's side, one per line.

process to the end of the file

 

Output

The number of triangles in the equilateral triangle, one per line.

 

Sample Input

   
   
   
   

    
    
    
    1
2
3
   
   
   
   

 

Sample Output

   
   
   
   

    
    
    
    1
5
13
   
   
   
   

 

 

递推：将前一个的三角形数+新增的两层大小的三角形数+新增的三层以上的三角形数就得出现在的三角形总数

 

#include <stdio.h>

int main()
{
    __int64 a[505];
    int i;
    int k;
    a[1] = 1;
    for(i = 2;i<=500;i++)
    {
        a[i] = a[i-1]+i*(i+1)/2;
        k = i-1;
        while(k>0)
        {
            a[i]+=k;
            k-=2;
        }
    }
    while(~scanf("%d",&k))
    {
        printf("%I64d\n",a[k]);
    }

    return 0;
}