48. Rotate Image

Rotate an n × n matrix 90° clockwise in place.

Work ring by ring from the outside in. Each position on a ring trades places with three others in a 4-cycle: top → right → bottom → left → top.

1function rotate(matrix: number[][]): void {
2  const n = matrix.length;
3  for (let layer = 0; layer < n >> 1; layer++) {
4    const first = layer, last = n - 1 - layer;
5    for (let i = first; i < last; i++) {
6      const off = i - first;
7      const top = matrix[first][i];
8      matrix[first][i] = matrix[last - off][first]; // left → top
9      matrix[last - off][first] = matrix[last][last - off]; // bottom → left
10      matrix[last][last - off] = matrix[i][last]; // right → bottom
11      matrix[i][last] = top;
12    }
13  }
14}

matrix

1	2	3	4
5	6	7	8
9	10	11	12
13	14	15	16

Ring 0: rows/cols 0..3.

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