float minDistances2[BallNum][2]; // 記錄某個小球，距離它最近的小球的距離，這個小球的下標
        for (i=0;i<BallNum;i++)
        {
            minDistances2[i][0] = 9999999;
            minDistances2[i][1] = -1;
        }

        // 求解所有小球兩兩之間的距離平方
        for (i=0;i<BallNum;i++)
        {
            for (j=0;j<BallNum;j++)
            {
                if
 
 (i!=j) // 自己和自己不需要比
                {
                    float dist2;
                    dist2 = (ball_x[i] - ball_x[j])*(ball_x[i] - ball_x[j])
                        + (ball_y[i] - ball_y[j])*(ball_y[i] - ball_y[j]);
                    if (dist2<minDistances2[i][0])
                    {
                        minDistances2[i][0
 
] = dist2;
                        minDistances2[i][1] = j;
                    }
                }
            }
        }   

        // 判斷球之間是否碰撞
        for (i=0;i<BallNum;i++)
        {
            if (minDistances2[i][0]<=4*radius*radius) // 最小距離小於閾值，發生碰撞
            {
                j = minDistances2[i][1
 
];
                // 交換速度             
                int temp;
                temp = ball_vx[i]; ball_vx[i] = ball_vx[j]; ball_vx[j] = temp;
                temp = ball_vy[i]; ball_vy[i] = ball_vy[j]; ball_vy[j] = temp;

                minDistances2[j][0] = 999999999; // 避免交換兩次速度，又回去了
                minDistances2[j][1] = -1;
            }
        }