Checkerboard Generator

Create a checker board generator, which takes as inputs n and 2 elements to generate an n x n checkerboard with those two elements as alternating squares.

Examples

checker_board(2, 7, 6) ➞ [
  [7, 6],
  [6, 7]
]

checker_board(3, "A", "B") ➞ [
  ["A", "B", "A"],
  ["B", "A", "B"],
  ["A", "B", "A"]
]

checker_board(4, "c", "d") ➞ [
  ["c", "d", "c", "d"],
  ["d", "c", "d", "c"],
  ["c", "d", "c", "d"],
  ["d", "c", "d", "c"]
]

checker_board(4, "c", "c") ➞ "invalid"

Notes

• Both elements can be either strings or integers.
• The first element should be on the upper left corner of the checker board. e.g. "c", not "d" should be element [0][0] for example 3.
• Return "invalid" if both inputs are identical (see example 4).