Rotate Matrix

Here is another Google interview question (http://www.careercup.com/question?id=14891673):

Given a matrix represented as int[n][n], rotate it 90 degrees clockwise in-place. (In-place means minimal extra memory to be used, i.e. don't make a new array to copy into). Rotate clockwise means top-row becomes right-column, right column becomes bottom-row etc.

In the solution uses one extra integer variable to rotate elements in each iteration. There are two for loops - the outer one deals with one layer of the matrix per iteration, while the inner one deals with rotation of the elements of the layers.

void rotateMatrix(int a[][]) {
    int n = a.length;
    if (n <= 1) {
        return; // nothing to do
    }

    /* layers */
    for (int i = 0; i < n / 2; i++) {
        /* elements */
        for (int j = i; j < n - i - 1; j++) {
            int saved = a[i][j];
            a[i][j] = a[n - j - 1][i];
            a[n - j - 1][i] = a[n - 1 - i][n - 1 - j];
            a[n - 1 - i][n - 1 - j] = a[j][n - 1 - i];
            a[j][n - 1 - i] = saved;
        }
    }
}

In the picture below you can see the different layers in different colours. It shows what happens after each iteration of the outer for loop.