See this article for in-place matrix transposition; also google for "in-place matrix transposition". It can be easily adapted to perform rotation by 90 degrees. To transpose square matrices, you just interchange b[i][j] with b[j][i] where b[k][l] is a[n*k+l]. On nonsquare matrices, it's considerably more difficult. "Without any extra space" is a rather strong requirement, maybe you meant O(1) space? (assuming integers are fixed size) Implementation in C++: here.
