Ukuran board sudoku 9 x 9
Langkah pertama membuat method checkBoard() terlebih dahulu, yaitu
1. cek secara vertikal & horizontal
2. cek secara grid

CEK BOARD  METHOD checkBoard()
FOR i = 0 UNTIL i = 8 AND i++
  FOR j = 0 UNTIL j = 8 AND j++
    IF borads[i][j]
      SET isPerp = checkPerp(i, j, boards[i][j])  // CEK PERPENDICULAR
      SET isGrid = checkGrid(i, j, boards[i][j])  // CEK GRID
      IF !isPerp || !isGrid
        RETURN FALSE
      END IF
    END IF
  END FOR
END FOR
RETURN TRUE


CEK PERPENDICULAR  METHOD checkPerp(i, j, fill)
FOR n = 0 UNTIL n = 8 AND n++
  // Cek Horizontal
  IF board[i][n] === fill && j !== n
    RETURN FALSE
  END IF

  // Cek Vertikal
  IF board[n][j] === fill && i !== n
    RETURN FALSE
  END IF
END FOR


CEK GRID  METHOD checkGrid(i, j, fill)
// NORMALISASI i
IF i == 0 || i == 1 || i == 2
  SET i = 0
EN IF

IF i == 3 || i == 4 || i == 5
  SET i = 3
EN IF

IF i == 6 || i == 7 || i == 8
  SET i = 6
EN IF

// NORMALISASI j
IF j == 0 || j == 1 || j == 2
  SET j = 0
EN IF

IF j == 3 || j == 4 || j == 5
  SET j = 3
EN IF

IF j == 6 || j == 7 || j == 8
  SET j = 6
EN IF

// CEK GRID
FOR n = i UNTIL n <= i + 2 AND n++
  FOR m = j UNTIL j <= j + 2 AND m++
    IF n != i && m != j && board[n][m] !== fill
      RETURN FALSE
    END IF
  END FOR
END FOR
RETURN TRUE

Langkah ke-2 yaitu membuat method solve() dengan mengimplementasikan backtrack
*backtrack adalah sebuah cara untuk menyimpan setiap langkah / proses atau dengan kata lain undo

// backtrack denga  format
// [
//    {
//      i: 0,
//      j: 0,
//      val: 5
//    }
// ]
SET isInvalidBoard = false
SET histories = []
checkBoard()

FOR i = 0 UNTIL i = 8
  FOR j = 0 UNTIL j = 8

    IF board[i][j] === null
    |   // ISI VALUE ANTARA 1 - 9
    |   FOR val = 1 UNTIL val = 9 AND val++
    |     SET isPerp = checkPerp(i, j,val)  // CEK PERPENDICULAR
    |     SET isGrid = checkGrid(i, j,val)  // CEK GRID
    |
    |     IF isPerp && isGrid
    |     |   SET board[i][j] = val
    |     |   PUSH {i, j, val} TO histories
    |     ELSE IF val === 9
    |     |   // DILAKUKAN BACKTRACK (cari yang valuenya < 9)
    |     |   SET FLAG = TRUE
    |     |   WHILE FLAG
    |     |     SET back = POP histories
    |     |     IF back
    |     |     |   IF back.val < 9
    |     |     |   |   SET FLAG = FALSE
    |     |     |   |   SET i = back.i
    |     |     |   |   SET j = back.j
    |     |     |   |   SET board[i][j] = board.val + 1
    |     |     |   ELSE
    |     |     |   |   SET board[back.i][back.j] = NULL
    |     |     |   END IF
    |     |     ELSE
    |     |     |   SET FLAG = FALSE
    |     |     |   SET isInvalidBoard = TRUE
    |     |     |   BREAK ALL LOOPING
    |     |     END IF
    |     |   END WHILE
    |     |
    |     END IF
    |     IF !BACKTRACK j++
    |  END FOR
    END IF
    IF !BACKTRACK i++
  END FOR
END FOR