Create Rotate Image.cpp

Basic Idea: Swap upper left corner of the matrix.
swap rule:
i,j         -> j, n-i-1
j,n-i-1     -> n-i-1,n-j-1
n-i-1,n-j-1 -> n-j-1,i
n-j-1,i     -> i,j
Note that never swap on the same i,j twice. My solution is:
1. if n is even number, we swap all points in square {[0,0],[n/2,n/2]}
2. if n is odd number, we swap all points in rectangle {[0,0],[n/2 + 1, n/2}. Central point can be ignored.