Place a digit from 1-4 into each cell, exactly one digit per cell. Adjacent cells (including diagonally-adjacent cells) must not contain the same digit. Some digits are already given for you.
(Rule and example from PGP IB)
- Numbers not Touching: From WPC 2018/Round 5. Normal Different Neighbours rules apply. Additionaly, 4 marked tetrominoes contain different numbers.
History of the Puzzle
Invented by Vladimir Portugalov (Belarus) in 2006. First appeared on WPC 2006/Part 1.