Write the function knightsTour(rows, cols) that takes a non-negative int n and returns a 2d list of a knight's tour on a rows-by-cols board, or None if this does not exist. Following these instructions, knightsTour(4, 5) should return a legal 4x5 knight's tour, such as this one:

[ 1, 14, 5, 18, 7] [10, 19 8 13 4] [15 2, 11, 6, 17] [20 9, 16 3, 12]