How to rotate a matrix 90 degrees without using any extra space?

Possible Duplicate:
Algorithm to rotate an image 90 degrees in place? (No extra memory)

By saying 90 degrees i mean to say if:

A = {1,2,3,
     4,5,6,
     7,8,9}

then after 90 degree rotation A becomes:

A = {7,4,1,
     8,5,2,
     9,6,3}