54. Spiral Matrix

Return all elements of an m × n matrix in spiral order, going clockwise from the top-left corner.

Walk right, down, left, up in turn. Turn clockwise whenever the next cell is outside the grid or already visited.

1function spiralOrder(matrix: number[][]): number[] {
2  const m = matrix.length, n = matrix[0].length, out: number[] = [];
3  const seen = matrix.map((row) => row.map(() => false));
4  const dirs = [[0, 1], [1, 0], [0, -1], [-1, 0]];
5  let r = 0, c = 0, d = 0;
6  for (let k = 0; k < m * n; k++) {
7    out.push(matrix[r][c]); seen[r][c] = true;
8    const [nr, nc] = [r + dirs[d][0], c + dirs[d][1]];
9    if (nr < 0 || nc < 0 || nr >= m || nc >= n || seen[nr][nc]) d = (d + 1) % 4;
10    r += dirs[d][0]; c += dirs[d][1];
11  }
12  return out;
13}

dir = right

matrix

1	2	3	4
5	6	7	8
9	10	11	12

output

empty

Start at (0, 0) heading right.

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