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

123

 

Sample Output

1513

 

 

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;}