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Cube Notations
To solve the puzzle we need to rotate the faces, so let's mark them with letters: F(front), R(right), L(left), D(down). A letter alone means a clockwise rotation while reverse turns are marked with an apostrophe (F') and double turns are marked with 2 (F2).
An example algorithm: F R' U2 (turn the front face clockwise, then the right counterclockwise and do a 180 degree turn of the up face)
Note: if you are already aware about 3x3x3 cube notations then
there is not difference between 2x2x2 and 3x3x3 cube notations.