private int[][] directions = new int[][]{{0, 1}, {1, 0}, {0, -1}, {-1, 0}};

public int[][] generateMatrix(int n) {

int[][] res = new int[n][n];

int i = 1;

int d = 0;

int x = 0;

int y = 0;

int END = n * n;

while (i <= END) {

res[x][y] = i;

int nx = x + directions[d][0];

int ny = y + directions[d][1];

if (nx < 0 || ny < 0 || nx >= n || ny >= n || res[nx][ny] != 0) {

d = (d + 1) % 4;

nx = x + directions[d][0];

ny = y + directions[d][1];

}

x = nx;

y = ny;

i++;

}

return res;

}

public int[][] generateMatrix2(int n) {

if (n == 0) return new int[0][0];

int[][] matrix = new int[n][n];

boolean[][] filled = new boolean[n][n];

int[] dx = new int[]{0, 1, 0, -1};

int[] dy = new int[]{1, 0, -1, 0};

int x = 0;

int y = 0;

int d = 0;

for (int i=1; i<=n*n; i++) {

matrix[x][y] = i;

filled[x][y] = true;

int xx = x + dx[d];

int yy = y + dy[d];

if (xx >= 0 && xx < n && yy >= 0 && yy < n && !filled[xx][yy]) {

x = xx;

y = yy;

} else {

d = (d + 1) % 4;

x += dx[d];

y += dy[d];

}

}

return matrix;

}