编写一个字符界面的五子棋人机对战程序。

好的，下面是一个简单的字符界面的五子棋人机对战程序，使用 Python 语言编写：

import random

# 定义棋盘大小
SIZE = 15

# 定义棋子类型
NONE = 0
BLACK = 1
WHITE = 2

# 初始化棋盘
board = [[NONE for i in range(SIZE)] for j in range(SIZE)]

# 判断某个位置是否在棋盘内
def is_valid(x, y):
    return x >= 0 and x < SIZE and y >= 0 and y < SIZE

# 判断某个位置是否为空
def is_empty(x, y):
    return board[x][y] == NONE

# 判断某个位置是否为黑子
def is_black(x, y):
    return board[x][y] == BLACK

# 判断某个位置是否为白子
def is_white(x, y):
    return board[x][y] == WHITE

# 判断是否有五子连珠
def has_five_in_a_row(color, x, y):
    count = 1
    for i in range(1, 5):
        if is_valid(x+i, y) and color == board[x+i][y]:
            count += 1
        else:
            break
    if count >= 5:
        return True
    for i in range(1, 5):
        if is_valid(x-i, y) and color == board[x-i][y]:
            count += 1
        else:
            break
    if count >= 5:
        return True
    count = 1
    for i in range(1, 5):
        if is_valid(x, y+i) and color == board[x][y+i]:
            count += 1
        else:
            break
    if count >= 5:
        return True
    for i in range(1, 5):
        if is_valid(x, y-i) and color == board[x][y-i]:
            count += 1
        else:
            break
    if count >= 5:
        return True
    count = 1
    for i in range(1, 5):
        if is_valid(x+i, y+i) and color == board[x+i][y+i]:
            count += 1
        else:
            break
    if count >= 5:
        return True
    for i in range(1, 5):
        if is_valid(x-i, y-i) and color == board[x-i][y-i]:
            count += 1
        else:
            break
    if count >= 5:
        return True
    count = 1
    for i in range(1, 5):
        if is_valid(x+i, y-i) and color == board[x+i][y-i]:
            count += 1
        else:
            break
    if count >= 5:
        return True
    for i in range(1, 5):
        if is_valid(x-i, y+i) and color == board[x-i][y+i]:
            count += 1
        else:
            break
    if count >= 5:
        return True
    return False

# 判断是否已经下满棋子
def is_full():
    for i in range(SIZE):
        for j in range(SIZE):
            if board[i][j] == NONE:
                return False
    return True

# 打印棋盘
def print_board():
    print(" ", end="")
    for i in range(SIZE):
        print(" %d" % i, end="")
    print("")
    for i in range(SIZE):
        print("%d " % i, end="")
        for j in range(SIZE):
            if board[i][j] == NONE:
                print(". ", end="")
            elif board[i][j] == BLACK:
                print("X ", end="")
            elif board[i][j] == WHITE:
                print("O ", end="")
        print("")

# 人机对战
def play():
    print("欢迎来到五子棋游戏！")
    print("您将执黑先行，电脑将执白后行。")
    print("请输入您的下棋坐标，例如 3,4。")
    while True:
        print_board()
        x, y = input("请输入您的下棋坐标：").split(",")
        x = int(x)
        y = int(y)
        if not is_valid(x, y):
            print("输入的坐标不合法，请重新输入！")
            continue
        if not is_empty(x, y):
            print("该位置已经有棋子，请重新输入！")
            continue
        board[x][y] = BLACK
        if has_five_in_a_row(BLACK, x, y):
            print_board()
            print("您赢了！")
            break
        if is_full():
            print_board()
            print("平局！")
            break
        print("电脑正在思考中......")
        while True:
            x = random.randint(0, SIZE-1)
            y = random.randint(0, SIZE-1)
            if is_empty(x, y):
                break
        board[x][y] = WHITE
        if has_five_in_a_row(WHITE, x, y):
            print_board()
            print("电脑赢了！")
            break
        if is_full():
            print_board()
            print("平局！")
            break

# 运行程序
play()

该程序使用随机算法实现电脑下棋，可以适当增加算法复杂度以提高游戏难度。此外，也可以使用其他算法实现更加智能的电脑下棋，例如 AlphaGo 等。