Question

I am making a TicTacToe game in Java I am required to use a two-dimensional array and to use recursion to check if there is a winner.

I feel like I could easily do a non-recursive check for a winner, but I don’t know where to start if I were to use recursion to do it because I am very new at recursion. Could someone guide me on where to start the process of such an algorithm?

Answer 1

Assuming your board looks something like:

            |            |
 cell[0][0] | cell[1][0] | cell[2][0]
            |            |
------------+------------+------------
            |            |
 cell[0][1] | cell[1][1] | cell[2][1]
            |            |
------------+------------+------------
            |            |
 cell[0][2] | cell[1][2] | cell[2][2]
            |            |

One way is to simply check adjacent cells recursively (in a single direction). For example (pseudo-code):

def checkSame (val, cellX, cellY. deltaX, deltaY):
    # No winner if check value is empty.

    if val == empty: return false

    # Winner if we've gone off edge. No need to worry about < 0
    # since one direction is always ascending but I've left it
    # in anyway.

    if cellX > 2 or cellY > 2: return true
    if cellX < 0 or cellY < 0: return true

    # No winner if piece has changed.

    if cell[cellX][cellY] != val: return false

    # Otherwise use recursion to check next one.

    return checkSame (val, cellX + deltaX, cellY + deltaY, deltaX, deltaY)

Then, we just have to check the eight possible start-point/direction values:

# Check rows.

if checkSame (cell[0][0], 0, 0, 1, 0): return true
if checkSame (cell[0][1], 0, 1, 1, 0): return true
if checkSame (cell[0][2], 0, 2, 1, 0): return true

# Check columns.

if checkSame (cell[0][0], 0, 0, 0, 1): return true
if checkSame (cell[1][0], 1, 0, 0, 1): return true
if checkSame (cell[2][0], 2, 0, 0, 1): return true

# Check diagonals.

if checkSame (cell[0][0], 0, 0, 1, 1): return true
return checkSame (cell[0][2], 0, 2, 1, -1)

Now, granted that’s a fairly limited (and contrived) use of recursion but, as you say, it’s not really a situation suited to recursion anyway. Far better is just to use eight if statements, provided you’re not planning of expanding this to more than standard 3×3 tic-tac-toe.