TicTacToe

The classic Tic-Tac-Toe game (also called Noughts and Crosses) or Xs and Os is a paper-and-pencil game for two players, X and O, who take turns marking the spaces in a 3×3 grid. The player who succeeds in placing three of their marks in a horizontal, vertical, or diagonal row is the winner.

In this code, I've used minimax algorithm to help the computer where to go for the next move and win the puzzle.

Combinatorics :

When considering only the state of the board, and after taking into account board symmetries (i.e. rotations and reflections), there are only 138 terminal board positions. A combinatorics study of the game shows that when "X" makes the first move every time, the game is won as follows :

• 91 distinct positions are won by (X)
  
• 44 distinct positions are won by (O)
  
• 3 distinct positions are drawn (often called a "cat's game")

Pseudocode

function minimax(node, depth, isMaximizingPlayer, alpha, beta):

    if node is a leaf node :
        return value of the node
    
    if isMaximizingPlayer :
        bestVal = -INFINITY 
        for each child node :
            value = minimax(node, depth+1, false, alpha, beta)
            bestVal = max( bestVal, value) 
            alpha = max( alpha, bestVal)
            if beta <= alpha:
                break
        return bestVal

    else :
        bestVal = +INFINITY 
        for each child node :
            value = minimax(node, depth+1, true, alpha, beta)
            bestVal = min( bestVal, value) 
            beta = min( beta, bestVal)
            if beta <= alpha:
                break
        return bestVal
        
// Calling the function for the first time.
minimax(0, 0, true, -INFINITY, +INFINITY)

Minimax Algorithm Visualisation

Instructions to run:

Windows

1. Install a cpp compiler
2. Open commmand prompt in the folder where program is saved
3. g++ ai.cpp then ./a

Linux

1. Install a cpp compiler
2. Open terminal in the folder where program is saved
3. g++ ai.cpp then ./a.out