打印NxN的矩阵

找出规律，并打印出一个NxN的矩阵，规律就是从首坐标开始顺时针依次增大：

 #include<iostream>
 #include<vector>

 using namespace std;

 int main(void)
 {

     int N;
     cout << "Input a num:" << endl;
     cin >> N;
     while (N < )
     {
         cout << "invalid input!!!" << endl;
         cout << "input again!!!" << endl;
         cin >> N;
     }
     if (N == )
     {
         cout << ;
         return ;
     }

     vector<vector<int> > p(N);
     for (int i = ; i < N; i++)
     {
         p[i].resize(N);//设置N*N的数组
     }
     int k = ;
     for (int j = ; j < N / ;j++)
     {
         for (int i = j; i < N - j; i++)
         {
             p[j][i] = k;
             k++;
         }
         for (int i =  + j; i < N - j; i++)
         {
             p[i][N - j - ] = k;
             k++;
         }
         for (int i = N -  - j; i >= j; i--)
         {
             p[N -  - j][i] = k;
             k++;
         }
         for (int i = N - -j; i >j; i--)
         {
             p[i][j] = k;
             k++;
         }
     }
     if (N %  == )
     {
         p[N / ][N / ] = k;
     }

     for (int i = ; i < N; i++)
     {
         for (int j = ; j < N; j++)
         {
             cout << p[i][j] << "\t";
         }
         cout << endl;
     }
     return ;
 }