Rotate Matrix By 90 Degree

Problem description: Given an N X N integer matrix, rotate it bye 90 degrees 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.
eg.

[1,  2, 3, 4]                   [9, 6, 9, 1]
[9, 8, 5, 6]         –>         [2, 5, 8, 2]
[6, 5, 3, 7]                    [6, 3, 5, 3]
[9, 2, 6, 8]                    [8, 7, 6, 4]

Solution:

The idea is to do a “four-way” swap variable, we need to move the values from top -> right, right -> bottom, bottom -> left and left -> top, can we do it by using only one extra variable? Yes, we can.

At each layer we are gonna loop through the elements and swap them as follows:
1.- Save the ith element in the top array in a temporary variable (in our example the top array is [1 2 3 4] ).
2.- Move the ith element from left to top.
3.- Move the ith element from bottom to left.
4.- Move the ith element from right to bottom.
5.- Save the value of our temporary variable in the ith position in the right array.

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;
        }
    }
}