This is one of my favorite Sudoku type. This puzzle type appeared in Oğuz Atay Puzzle Contest (OAPC)-2 championship and The Last Sudoku Competition Before World Sudoku Championship (TLSCBWSC-1)
Fill the grid with digits 1~4, so that each digit appears exactly once in every row, column and every 3x3 spiral. Digits should be placed orderly in the spirals, from the entrance to the center. The numbers outside the grid indicate the first seen number from that direction.
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