https://leetcode.cn/problems/spiral-matrix-ii/description/

class Solution {
public:
    vector<vector<int>> generateMatrix(int n) {
        int x = 0, y = 0;
        // 重要，矩陣四邊的前進方向
        int dx[4] = {0, 1, 0, -1}, dy[4] = {1, 0, -1, 0}, dxy_iter = 0;
        // 重要，先測試下一步的坐標，如出界或重復，則改變前進方向
        int nx = 0, ny = 0;
        vector<vector<int>> matrix = vector<vector<int>>(n, vector<int>(n, 0));
        vector<vector<bool>> visited = vector<vector<bool>>(n, vector<bool>(n, false));
        for (int i=1; i<=n*n; ++i) {
            // cout << x << " " << y << endl;
            matrix[x][y] = i;
            visited[x][y] = true;
            nx = x + dx[dxy_iter];
            ny = y + dy[dxy_iter];
            if (nx>=n || ny>=n || nx<0 || ny<0 || visited[nx][ny]) {
                dxy_iter = (dxy_iter + 1) % 4;
                x = x + dx[dxy_iter];
                y = y + dy[dxy_iter];
            } else {
                x = nx;
                y = ny;
            }
        }
        return matrix;
    }
};