Finish HackerRank Tic-Tac-Toe Min-Max challenge

Author: andreimaximov

.gitignore

@@ -1,5 +1,5 @@
 out
-*.class
 __pycache__
+.vscode
+*.class
 *.swp
-

hacker-rank/artificial-intelligence/alpha-beta-pruning/tic-tac-toe/README.md

@@ -0,0 +1,4 @@
+# README
+
+[https://www.hackerrank.com/challenges/tic-tac-toe](https://www.hackerrank.com/challenges/tic-tac-toe)
+

hacker-rank/artificial-intelligence/alpha-beta-pruning/tic-tac-toe/solution.py

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+#!/usr/bin/env python
+
+import sys
+
+# Map from max player -> values for winners. If X' is the max player, then
+# a tie with '_' winning is better than 'O' winning but worse than 'X' winning.
+VALUES = {'X': {'O': 0, '_': 1, 'X': 2},
+          'O': {'X': 0, '_': 1, 'O': 2}}
+
+
+def main():
+    player = raw_input()
+    board = [list(raw_input()) for _ in range(3)]
+    r, c = minimax(board, player, player)[1]
+    print('%s %s' % (r, c))
+
+
+def minimax(board, curr_player, max_player):
+    '''Runs min-max on the board.'''
+    if finished(board):
+        w = winner(board)
+        return VALUES[max_player][w], None
+
+    if curr_player == 'X':
+        value, next_player = VALUES[max_player]['O'], 'O'
+    else:
+        value, next_player = VALUES[max_player]['X'], 'X'
+
+    move = None
+
+    for r, c in options(board):
+        board[r][c] = curr_player
+        value_ = minimax(board, next_player, max_player)[0]
+        board[r][c] = '_'
+        if (curr_player == max_player and value_ >= value) or \
+           (curr_player != max_player and value_ <= value):
+            value, move = value_, (r, c)
+
+    return value, move
+
+
+def options(board):
+    '''Returns a generator of available moves.'''
+    for r in range(3):
+        for c in range(3):
+            if board[r][c] == '_':
+                yield (r, c)
+
+
+def finished(board):
+    '''Returns if the game is finished or not.'''
+    if winner(board) != '_':
+        return True
+    for r in board:
+        for x in r:
+            if x == '_':
+                return False
+    return True
+
+
+def winner(board):
+    '''Returns the current winner, if one exists.'''
+    for i in range(3):
+        # Check rows...
+        if board[i][0] != '_' and board[i][0] == board[i][1] == board[i][2]:
+            return board[i][0]
+        # Check cols...
+        if board[0][i] != '_' and board[0][i] == board[1][i] == board[2][i]:
+            return board[0][i]
+
+    # Check diags...
+    if board[0][0] != '_' and board[0][0] == board[1][1] == board[2][2]:
+        return board[0][0]
+
+    if board[2][0] != '_' and board[2][0] == board[1][1] == board[0][2]:
+        return board[2][0]
+
+    return '_'
+
+
+if __name__ == '__main__':
+    main()