给我一段c++代码，完成五子棋游戏，要求包含三种游戏模式：人人对战，人机对战（机器先下），人机对战（人先下））

以下是一个简单的五子棋游戏的 C++ 代码，包含了三种游戏模式：

#include<iostream>
#include<cstring>
#include<cstdlib>
using namespace std;

const int MAXN = 20;
const int INF = 1e9;

int n;
char mp[MAXN][MAXN];
int val[MAXN][MAXN];
int dx[4] = {1, 0, -1, 1};
int dy[4] = {0, 1, 1, 1};

void init(){
    for(int i = 0; i < n; i++){
        for(int j = 0; j < n; j++){
            mp[i][j] = ' ';
            val[i][j] = 0;
        }
    }
}

void draw_map(){
    for(int i = 0; i < n; i++){
        cout << i+1 << " ";
        for(int j = 0; j < n; j++){
            cout << mp[i][j] << " ";
        }
        cout << endl;
    }
    cout << "  ";
    for(int i = 1; i <= n; i++){
        cout << i << " ";
    }
    cout << endl;
}

inline bool check(int x, int y){
    if(x < 0 || x >= n || y < 0 || y >= n) return false;
    if(mp[x][y] != ' ') return false;
    return true;
}

int evaluation(){
    int res = 0;
    for(int i = 0; i < n; i++){
        for(int j = 0; j < n; j++){
            if(mp[i][j] == 'O'){
                for(int k = 0; k < 4; k++){
                    int cnt = 1;
                    int nx = i + dx[k], ny = j + dy[k];
                    while(check(nx, ny) && mp[nx][ny] == 'O'){
                        cnt++; nx += dx[k]; ny += dy[k];
                    }
                    if(cnt >= 5) return INF;
                    res += val[cnt][0];
                }
            }
            if(mp[i][j] == 'X'){
                for(int k = 0; k < 4; k++){
                    int cnt = 1;
                    int nx = i + dx[k], ny = j + dy[k];
                    while(check(nx, ny) && mp[nx][ny] == 'X'){
                        cnt++; nx += dx[k]; ny += dy[k];
                    }
                    if(cnt >= 5) return -INF;
                    res -= val[cnt][0];
                }
            }
        }
    }
    return res;
}

int alpha_beta(int dep, int alpha, int beta, bool flag){
    if(dep == 0) return evaluation();
    int res = flag ? -INF : INF;
    for(int i = 0; i < n; i++){
        for(int j = 0; j < n; j++){
            if(mp[i][j] == ' '){
                if(flag){
                    mp[i][j] = 'O';
                    res = max(res, alpha_beta(dep-1, alpha, beta, false));
                    alpha = max(alpha, res);
                }
                else{
                    mp[i][j] = 'X';
                    res = min(res, alpha_beta(dep-1, alpha, beta, true));
                    beta = min(beta, res);
                }
                mp[i][j] = ' ';
                if(alpha >= beta) return res;
            }
        }
    }
    return res;
}

void player_vs_player(){
    cout << "Player 1 is O, Player 2 is X." << endl;
    int cnt = 0;
    while(cnt < n*n){
        int x, y;
        if(cnt%2 == 0) cout << "Player 1's turn: ";
        else cout << "Player 2's turn: ";
        cin >> x >> y;
        x--; y--;
        if(check(x, y)){
            if(cnt%2 == 0) mp[x][y] = 'O';
            else mp[x][y] = 'X';
            draw_map();
            int res = evaluation();
            if(res == INF){
                cout << "Player 1 wins!" << endl;
                return;
            }
            if(res == -INF){
                cout << "Player 2 wins!" << endl;
                return;
            }
            cnt++;
        }
        else{
            cout << "Invalid input, please try again." << endl;
        }
    }
    cout << "It's a tie!" << endl;
}

void player_vs_computer(bool flag){
    cout << (flag ? "You are O, computer is X." : "You are X, computer is O.") << endl;
    int cnt = 0;
    while(cnt < n*n){
        int x, y;
        if(cnt%2 == flag){
            cout << "Your turn: ";
            cin >> x >> y;
            x--; y--;
            if(check(x, y)){
                mp[x][y] = flag ? 'O' : 'X';
                draw_map();
                int res = evaluation();
                if(res == INF){
                    cout << "You win!" << endl;
                    return;
                }
                if(res == -INF){
                    cout << "Computer wins!" << endl;
                    return;
                }
                cnt++;
            }
            else{
                cout << "Invalid input, please try again." << endl;
            }
        }
        else{
            int maxv = -INF;
            for(int i = 0; i < n; i++){
                for(int j = 0; j < n; j++){
                    if(mp[i][j] == ' '){
                        mp[i][j] = flag ? 'X' : 'O';
                        int tmp = alpha_beta(5, -INF, INF, !flag);
                        mp[i][j] = ' ';
                        if(tmp > maxv){
                            maxv = tmp;
                            x = i; y = j;
                        }
                    }
                }
            }
            cout << "Computer's turn: " << x+1 << " " << y+1 << endl;
            mp[x][y] = flag ? 'X' : 'O';
            draw_map();
            int res = evaluation();
            if(res == INF){
                cout << "You win!" << endl;
                return;
            }
            if(res == -INF){
                cout << "Computer wins!" << endl;
                return;
            }
            cnt++;
        }
    }
    cout << "It's a tie!" << endl;
}

void computer_vs_player(bool flag){
    cout << (flag ? "Computer is O, you are X." : "Computer is X, you are O.") << endl;
    int cnt = 0;
    while(cnt < n*n){
        int x, y;
        if(cnt%2 == flag){
            int maxv = -INF;
            for(int i = 0; i < n; i++){
                for(int j = 0; j < n; j++){
                    if(mp[i][j] == ' '){
                        mp[i][j] = flag ? 'X' : 'O';
                        int tmp = alpha_beta(5, -INF, INF, !flag);
                        mp[i][j] = ' ';
                        if(tmp > maxv){
                            maxv = tmp;
                            x = i; y = j;
                        }
                    }
                }
            }
            cout << "Computer's turn: " << x+1 << " " << y+1 << endl;
            mp[x][y] = flag ? 'O' : 'X';
            draw_map();
            int res = evaluation();
            if(res == INF){
                cout << "Computer wins!" << endl;
                return;
            }
            if(res == -INF){
                cout << "You win!" << endl;
                return;
            }
            cnt++;
        }
        else{
            cout << "Your turn: ";
            cin >> x >> y;
            x--; y--;
            if(check(x, y)){
                mp[x][y] = flag ? 'X' : 'O';
                draw_map();
                int res = evaluation();
                if(res == INF){
                    cout << "Computer wins!" << endl;
                    return;
                }
                if(res == -INF){
                    cout << "You win!" << endl;
                    return;
                }
                cnt++;
            }
            else{
                cout << "Invalid input, please try again." << endl;
            }
        }
    }
    cout << "It's a tie!" << endl;
}

int main(){
    cout << "Please input the size of the chessboard (maximum 20): ";
    cin >> n;
    init();
    draw_map();
    while(true){
        cout << "Please choose the game mode: " << endl;
        cout << "1. Player vs Player" << endl;
        cout << "2. Player vs Computer (computer goes first)" << endl;
        cout << "3. Player vs Computer (player goes first)" << endl;
        int mode;
        cin >> mode;
        if(mode == 1){
            player_vs_player();
        }
        else if(mode == 2){
            player_vs_computer(true);
        }
        else if(mode == 3){
            computer_vs_player(true);
        }
        else{
            cout << "Invalid input, please try again." << endl;
        }
        cout << "Would you like to play again? (y/n)" << endl;
        char c;
        cin >> c;
        if(c != 'y' && c != 'Y') break;
        init();
        draw_map();
    }
    return 0;
}

在该代码中，我们使用了 alpha-beta 剪枝算法来实现了简单的 AI。注意到这个 AI 的效率并不高，对于较大的棋盘或深度，可能需要更高效的算法进行优化。