《数据结构与算法分析：C语言描述》复习——第十章“算法设计技巧”——Alpha-Beta剪枝

  1 // Optimization for Minimax game strategy, using Alpha-Beta Pruning.
  2 // You can watch over the ‘function_call_count‘ variable.
  3 #include <iostream>
  4 #include <vector>
  5 using namespace std;
  6 
  7 int function_call_count;
  8 
  9 bool computerWin(const vector<int> &board)
 10 {
 11     int i, j;
 12     
 13     for (i = 0; i < 3; ++i) {
 14         for (j = 0; j < 3; ++j) {
 15             if (board[i * 3 + j] != -1) {
 16                 break;
 17             }
 18         }
 19         if (j == 3) {
 20             return true;
 21         }
 22     }
 23     
 24     for (i = 0; i < 3; ++i) {
 25         for (j = 0; j < 3; ++j) {
 26             if (board[j * 3 + i] != -1) {
 27                 break;
 28             }
 29         }
 30         if (j == 3) {
 31             return true;
 32         }
 33     }
 34     
 35     if (board[0] == board[4] && board[4] == board[8] && board[8] == -1) {
 36         return true;
 37     }
 38     
 39     if (board[2] == board[4] && board[4] == board[6] && board[6] == -1) {
 40         return true;
 41     }
 42     
 43     return false;
 44 }
 45 
 46 bool humanWin(const vector<int> &board)
 47 {
 48     int i, j;
 49     
 50     for (i = 0; i < 3; ++i) {
 51         for (j = 0; j < 3; ++j) {
 52             if (board[i * 3 + j] != 1) {
 53                 break;
 54             }
 55         }
 56         if (j == 3) {
 57             return true;
 58         }
 59     }
 60     
 61     for (i = 0; i < 3; ++i) {
 62         for (j = 0; j < 3; ++j) {
 63             if (board[j * 3 + i] != 1) {
 64                 break;
 65             }
 66         }
 67         if (j == 3) {
 68             return true;
 69         }
 70     }
 71     
 72     if (board[0] == board[4] && board[4] == board[8] && board[8] == 1) {
 73         return true;
 74     }
 75     
 76     if (board[2] == board[4] && board[4] == board[6] && board[6] == 1) {
 77         return true;
 78     }
 79     
 80     return false;
 81 }
 82 
 83 bool fullBoard(const vector<int> &board)
 84 {
 85     for (int i = 0; i < 9; ++i) {
 86         if (board[i] == 0) {
 87             return false;
 88         }
 89     }
 90     
 91     return true;
 92 }
 93 
 94 void findComputerMove(vector<int> &board, int &best_move, int &result, 
 95     int alpha, int beta)
 96 {
 97     void findHumanMove(vector<int> &, int &, int &, int, int);
 98     int dc, i, response;
 99     
100     ++function_call_count;
101     best_move = -1;
102 
103     if (fullBoard(board)) {
104         result = 0;
105         return;
106     }
107     
108     if (computerWin(board)) {
109         result = -1;
110         return;
111     }
112     
113     result = alpha;
114     for (i = 0; i < 9 && result > beta; ++i) {
115         if (board[i] != 0) {
116             continue;
117         }
118         board[i] = -1;
119         findHumanMove(board, dc, response, result, beta);
120         board[i] = 0;
121         
122         if (best_move == -1 || response < result) {
123             result = response;
124             best_move = i;
125         }
126     }
127 }
128 
129 void findHumanMove(vector<int> &board, int &best_move, int &result, int alpha, 
130     int beta)
131 {
132     void findComputerMove(vector<int> &, int &, int &, int, int);
133     int dc, i, response;
134     
135     ++function_call_count;
136     best_move = -1;
137 
138     if (fullBoard(board)) {
139         result = 0;
140         return;
141     }
142     
143     if (humanWin(board)) {
144         result = 1;
145         return;
146     }
147     
148     result = beta;
149     for (i = 0; i < 9 && result < alpha; ++i) {
150         if (board[i] != 0) {
151             continue;
152         }
153         board[i] = 1;
154         findComputerMove(board, dc, response, alpha, result);
155         board[i] = 0;
156         
157         if (best_move == -1 || response > result) {
158             result = response;
159             best_move = i;
160         }
161     }
162 }
163 
164 void printBoard(const vector<int> &board)
165 {
166     cout << "  1 2 3" << endl;
167     int i, j;
168     
169     for (i = 0; i < 3; ++i) {
170         cout << i + 1;
171         for (j = 0; j < 3; ++j) {
172             cout << ‘ ‘;
173             switch(board[i * 3 + j]) {
174             case -1:
175                 cout << ‘X‘;
176                 break;
177             case 0:
178                 cout << ‘.‘;
179                 break;
180             case 1:
181                 cout << ‘O‘;
182                 break;
183             }
184         }
185         cout << endl;
186     }
187 }
188 
189 int main()
190 {
191     vector<int> board;
192     int n;
193     int result;
194     
195     board.resize(9, 0);
196     while (cin >> n) {
197         if (n < 0 || n >= 9 || board[n]) {
198             cout << "Invalid move" << endl;
199             continue;
200         }
201 
202         board[n] = 1;
203         printBoard(board);
204         if (humanWin(board)) {
205             cout << "You win." << endl;
206             break;
207         }
208         
209         if (fullBoard(board)) {
210             cout << "Draw." << endl;
211             break;
212         }
213         
214         result = 1;
215         function_call_count = 0;
216         findComputerMove(board, n, result, 1, -1);
217         cout << "Number of function calls: " << function_call_count << endl;
218         board[n] = -1;
219         printBoard(board);
220         if (computerWin(board)) {
221             cout << "Computer win." << endl;
222             break;
223         }
224         
225         if (fullBoard(board)) {
226             cout << "Draw." << endl;
227             break;
228         }
229     }
230     
231     return 0;
232 }