The idea is rotate(a[i][j]) = a[j][n-i-1];

...

```
public void rotate(int[][] matrix) {
int n = matrix.length;
if (n == 0) return;
for (int i = 0; i < n>>1; i++) {
for (int j = i; j < n-i-1; j++) {
int temp = matrix[n-j-1][i];
matrix[n-j-1][i] = matrix[n-i-1][n-j-1];
matrix[n-i-1][n-j-1] = matrix[j][n-i-1];
matrix[j][n-i-1] = matrix[i][j];
matrix[i][j] = temp;
}
}
}
```

...