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 < 0)
    {
        cout << "invalid input!!!" << endl;
        cout << "input again!!!" << endl;
        cin >> N;
    }
    if (N == 0)
    {
        cout << 0;
        return 0;
    }

    vector<vector<int> > p(N);
    for (int i = 0; i < N; i++)
    {
        p[i].resize(N);//设置N*N的数组
    }
    int k = 0;
    for (int j = 0; j < N / 2;j++)
    {
        for (int i = j; i < N - j; i++)
        {
            p[j][i] = k;
            k++;
        }
        for (int i = 1 + j; i < N - j; i++)
        {
            p[i][N - j - 1] = k;
            k++;
        }
        for (int i = N - 2 - j; i >= j; i--)
        {
            p[N - 1 - j][i] = k;
            k++;
        }
        for (int i = N - 2-j; i >j; i--)
        {
            p[i][j] = k;
            k++;
        }
    }
    if (N % 2 == 1)
    {
        p[N / 2][N / 2] = k;
    }

    for (int i = 0; i < N; i++)
    {
        for (int j = 0; j < N; j++)
        {
            cout << p[i][j] << "	";
        }
        cout << endl;
    }
    return 0;
}