博弈论中的MinMax搜索算法

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#include <iostream>
#include <vector>
using namespace std;

char board[3][3];

void init() {
    for (int i = 0; i < 3; i++)
        for (int j = 0; j < 3; j++)
            board[i][j] = '_';
}

void draw_board() {
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 3; j++)
            cout << board[i][j] << " ";
        cout << endl;
    }
    cout << endl;
}

int game_result(){
    for (int i = 0; i < 3; i++){
        int r =  board[i][0] +  board[i][1] + board[i][2];
        int c =  board[0][i] +  board[1][i] + board[2][i];
        if (r == 3 * 'X' || c == 3 * 'X') return 0; 
        if (r == 3 * 'O' || c == 3 * 'O') return 1;
    }

    int d1 = board[0][0] + board[1][1] + board[2][2];
    int d2 = board[0][2] + board[1][1] + board[2][0];
    if (d1 == 3 * 'X' || d2 == 3 * 'X') return 0;
    if (d1 == 3 * 'O' || d2 == 3 * 'O') return 1;
    return -1;
}

//评估函数：电脑赢：空格数+1, 玩家赢：-空格数-1,平局：0 
int eval() {
    int res = 0;
    for (int i = 0;i < 3;i++)
        for (int j = 0;j < 3;j++)
            if (board[i][j] == '_') res++;

    int flag = game_result();
    if (flag == 1)  return res + 1;
    if (flag == 0)  return -(res + 1);
    return 0;
}

//从当前状态开始DFS搜索最优的落子位置：电脑最大化收益，人最小化收益。
int MinMaxSearch(int& pos, int step) {      //step：记录层数,奇数层为电脑操作,偶数层为玩家操作  
    int val = step & 1 ? -100 : 100;        //奇数层取极大值,偶数层取极小值
    if (game_result() >= 0) return eval();  //游戏结束，评估分数

    vector<int> ava_pos;    //记录还有那些位置可以下棋
    for (int i = 0; i < 3; i++)
        for (int j = 0; j < 3; j++)
            if (board[i][j] == '_') ava_pos.push_back(i * 3 + j);
    if (ava_pos.size() == 0) return eval(); 

    for (int i = 0; i < ava_pos.size(); i++) {
        int x = ava_pos[i];
        int t = x;
        board[x / 3][x % 3] = (step & 1) ? 'O' : 'X';
        int son_val = MinMaxSearch(x, step + 1);
        board[t / 3][t % 3] = '_';

        if (step & 1) {
            if (val < son_val) {
                val = son_val;
                if (step == 1) pos = ava_pos[i];  //初始状态下的最优孩子节点（落子位置）。
            }
        }
        else  val = min(val, son_val);
    }
    return val;
}

int main() {
    init();

    for (int i = 1; i <= 9; i++) {
        //电脑下棋=============================================================
        cout << endl << "computer move:" << endl;
        int computer_pos; 
        MinMaxSearch(computer_pos, 1);
        board[computer_pos / 3][computer_pos % 3] = 'O';
        draw_board();

        //玩家下棋=============================================================
        printf("Select a position(1~9):");
        int x; cin >> x; x -= 1;
        while (1) {
            if (x > 0 && x < 10 && board[x / 3][x % 3] == '_') break;
            printf("The position is not available！please reselect.n");
            cin >> x;  x -= 1;
        }
        board[x / 3][x % 3] = 'X';
        draw_board();

        //判断游戏是否已经结束=================================================
        int Winer = game_result();
        if (Winer >= 0){
            Winer ? printf("You lose！n") : printf("You win！n");
            return 0;
        }
    }
    printf("平局n");

    return 0;
}