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：
 1 #include<iostream>
 2 #include<vector>
 3 
 4 using namespace std;
 5 
 6 int main(void)
 7 {
 8 
 9     int N;
10     cout << "Input a num:" << endl;
11     cin >> N;
12     while (N < 0)
13     {
14         cout << "invalid input!!!" << endl;
15         cout << "input again!!!" << endl;
16         cin >> N;
17     }
18     if (N == 0)
19     {
20         cout << 0;
21         return 0;
22     }
23 
24     vector<vector<int> > p(N);
25     for (int i = 0; i < N; i++)
26     {
27         p[i].resize(N);//设置N*N的数组
28     }
29     int k = 0;
30     for (int j = 0; j < N / 2;j++)
31     {
32         for (int i = j; i < N - j; i++)
33         {
34             p[j][i] = k;
35             k++;
36         }
37         for (int i = 1 + j; i < N - j; i++)
38         {
39             p[i][N - j - 1] = k;
40             k++;
41         }
42         for (int i = N - 2 - j; i >= j; i--)
43         {
44             p[N - 1 - j][i] = k;
45             k++;
46         }
47         for (int i = N - 2-j; i >j; i--)
48         {
49             p[i][j] = k;
50             k++;
51         }
52     }
53     if (N % 2 == 1)
54     {
55         p[N / 2][N / 2] = k;
56     }
57 
58     for (int i = 0; i < N; i++)
59     {
60         for (int j = 0; j < N; j++)
61         {
62             cout << p[i][j] << "	";
63         }
64         cout << endl;
65     }
66     return 0;
67 }