我已经实现了 Negamax，因为它可以在wikipedia上找到，其中包括 alpha/beta 修剪。
然而，它似乎有利于失败的举动，据我所知，这是一个无效的结果。
游戏是井字游戏，我已经抽象了大部分游戏玩法，因此应该很容易在算法中发现错误。
#include <list>
#include <climits>
#include <iostream>
//#define DEBUG 1
using namespace std;
struct Move {
int row, col;
Move(int row, int col) : row(row), col(col) { }
Move(const Move& m) { row = m.row; col = m.col; }
};
struct Board {
char player;
char opponent;
char board[3][3];
Board() { }
void read(istream& stream) {
stream >> player;
opponent = player == 'X' ? 'O' : 'X';
for(int row = 0; row < 3; row++) {
for(int col = 0; col < 3; col++) {
char playa;
stream >> playa;
board[row][col] = playa == '_' ? 0 : playa == player ? 1 : -1;
}
}
}
void print(ostream& stream) {
for(int row = 0; row < 3; row++) {
for(int col = 0; col < 3; col++) {
switch(board[row][col]) {
case -1:
stream << opponent;
break;
case 0:
stream << '_';
break;
case 1:
stream << player;
break;
}
}
stream << endl;
}
}
void do_move(const Move& move, int player) {
board[move.row][move.col] = player;
}
void undo_move(const Move& move) {
board[move.row][move.col] = 0;
}
bool isWon() {
if (board[0][0] != 0) {
if (board[0][0] == board[0][1] &&
board[0][1] == board[0][2])
return true;
if (board[0][0] == board[1][0] &&
board[1][0] == board[2][0])
return true;
}
if (board[2][2] != 0) {
if (board[2][0] == board[2][1] &&
board[2][1] == board[2][2])
return true;
if (board[0][2] == board[1][2] &&
board[1][2] == board[2][2])
return true;
}
if (board[1][1] != 0) {
if (board[0][1] == board[1][1] &&
board[1][1] == board[2][1])
return true;
if (board[1][0] == board[1][1] &&
board[1][1] == board[1][2])
return true;
if (board[0][0] == board[1][1] &&
board[1][1] == board[2][2])
return true;
if (board[0][2] == board [1][1] &&
board[1][1] == board[2][0])
return true;
}
return false;
}
list<Move> getMoves() {
list<Move> moveList;
for(int row = 0; row < 3; row++)
for(int col = 0; col < 3; col++)
if (board[row][col] == 0)
moveList.push_back(Move(row, col));
return moveList;
}
};
ostream& operator<< (ostream& stream, Board& board) {
board.print(stream);
return stream;
}
istream& operator>> (istream& stream, Board& board) {
board.read(stream);
return stream;
}
int evaluate(Board& board) {
int score = board.isWon() ? 100 : 0;
for(int row = 0; row < 3; row++)
for(int col = 0; col < 3; col++)
if (board.board[row][col] == 0)
score += 1;
return score;
}
int negamax(Board& board, int depth, int player, int alpha, int beta) {
if (board.isWon() || depth <= 0) {
#if DEBUG > 1
cout << "Found winner board at depth " << depth << endl;
cout << board << endl;
#endif
return player * evaluate(board);
}
list<Move> allMoves = board.getMoves();
if (allMoves.size() == 0)
return player * evaluate(board);
for(list<Move>::iterator it = allMoves.begin(); it != allMoves.end(); it++) {
board.do_move(*it, -player);
int val = -negamax(board, depth - 1, -player, -beta, -alpha);
board.undo_move(*it);
if (val >= beta)
return val;
if (val > alpha)
alpha = val;
}
return alpha;
}
void nextMove(Board& board) {
list<Move> allMoves = board.getMoves();
Move* bestMove = NULL;
int bestScore = INT_MIN;
for(list<Move>::iterator it = allMoves.begin(); it != allMoves.end(); it++) {
board.do_move(*it, 1);
int score = -negamax(board, 100, 1, INT_MIN + 1, INT_MAX);
board.undo_move(*it);
#if DEBUG
cout << it->row << ' ' << it->col << " = " << score << endl;
#endif
if (score > bestScore) {
bestMove = &*it;
bestScore = score;
}
}
if (!bestMove)
return;
cout << bestMove->row << ' ' << bestMove->col << endl;
#if DEBUG
board.do_move(*bestMove, 1);
cout << board;
#endif
}
int main() {
Board board;
cin >> board;
#if DEBUG
cout << "Starting board:" << endl;
cout << board;
#endif
nextMove(board);
return 0;
}
给出这个输入：
O
X__
___
___
算法选择将棋子放置在 0、1 处，造成一定的损失，对这个陷阱执行此操作（没有任何事情可以赢得或以平局告终）：
XO_
X__
___
我很确定游戏实现是正确的，但算法也应该是正确的。
编辑：更新evaluate和nextMove.
EDIT2：修复了第一个问题，但似乎仍然存在错误