Cacher is the code snippet organizer for pro developers

// g++ -o mat-spiral -std=c++11 mat-spiral.cpp
#include <iostream>
#include <vector>

using namespace std;

using ARRAY = vector<vector<int>>;

const ARRAY m5 {
    {  0,  1,  2,  3,  4 },
    {  5,  6,  7,  8,  9 },
    { 10, 11, 12, 13, 14 },
    { 15, 16, 17, 18, 19 },
    { 20, 21, 22, 23, 24 }
};

const ARRAY m10 {
    {  0,  1,  2,  3,  4,  5,  6,  7,  8,  9 },
    { 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 },
    { 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 },
    { 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 },
    { 40, 41, 42, 43, 44, 45, 46, 47, 48, 49 },
    { 50, 51, 52, 53, 54, 55, 56, 57, 58, 59 },
    { 60, 61, 62, 63, 64, 65, 66, 67, 68, 69 },
    { 70, 71, 72, 73, 74, 75, 76, 77, 78, 79 },
    { 80, 81, 82, 83, 84, 85, 86, 87, 88, 89 },
    { 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 }
};

// N x N matrix spiral
void compute(const ARRAY& mm) {
    auto N = mm.size();
    cout << "N " << N << endl;
    // N of spiral iteration
    for (auto i = 0; i < N; i++) {
        // top
        for (auto j = i; j < N; j++) {
            // reach to an end of right column
            cout << mm[i][j] << " ";
            if (j == N-1) {
                cout << endl;
                // from top to bottom
                for (int k = i+1; k < N; k++)
                    cout << mm[k][N-1] << " ";
                cout << endl;
                // from right to left
                for (int k = N-2; k >= i; k--)
                    cout << mm[N-1][k] << " ";
                cout << endl;
                // from bottom to top
                for (int k = N-2; k >= i+1; k--)
                    cout << mm[k][i] << " ";
                cout << endl;
            }
        }
        N--;
    }
}

int main() {
    compute(m5);
    cout << "-----" << endl;
    compute(m10);
    return 0;
}