逆时针的蛇形矩阵

#include <stdio.h>
int main(int argc, char *argv[])
{int a,d,j,i,n,t,r[100][100],ans=0;      
scanf("%d",&n);  
a=1;
j=1;
d=n;
t=n*n;     
do
{      
    for (j=d;j>=a;j--)         
    {
        r[d][j]=t--;
    }      
    for (i=(d-1);i>=a;i--)        
    {
        r[i][a]=t--;
    }      
    for (j=a+1;j<=(n-a);j++)        
    {
        r[a][j]=t--;
    }      
    for (i=a;i<=d-1;i++)         
    {
        r[i][d]=t--;
    }      
    a=a+1;
    d=d-1;     
}while (a!=n-1);     
if (n==3) r[2][2]=1 ;      
for (i=1;i<=n;i++)      
{  
    for (j=1;j<=n;j++)   
        printf("%4d   ",r[i][j]);         
    printf("\n");            
}
for (i=1;i<=n;i++)
{
    ans+=r[i][i];
    ans+=r[n+1-i][i];
}  
printf("%d",ans-1);  
}