关于keystate函数的问题,我只按了一次左方向键，他就像按了几次一样

  
  while(TRUE)
    {
        if(KEYDOWN(VK_LEFT))
             {
                 if(S[0].DIR==UP_DIR)
                 t.DIR=LEFT_DIR;             //上转左=左
                 else if(S[0].DIR==LEFT_DIR)
                 t.DIR=DOWN_DIR;              //左转左=下
                 else if(S[0].DIR==DOWN_DIR)
                 t.DIR=RIGHT_DIR;             //下转左=右
                 else
                 t.DIR=UP_DIR;                  //右转左=上            
             }
         else if(KEYDOWN(VK_RIGHT))
             {
                 if(S[0].DIR==UP_DIR)
                 t.DIR=RIGHT_DIR;         //上转右=右
                 else if(S[0].DIR==RIGHT_DIR)
                 t.DIR=DOWN_DIR;          //右转右=下
                 else if(S[0].DIR==DOWN_DIR)   
                 t.DIR=LEFT_DIR;             //下转右=左
                 else
                 t.DIR=UP_DIR;            //左转右=上     
             }
             else
             {
                 t.DIR=S[0].DIR;
                 delay(10);   
             }
             if(t.DIR==UP_DIR)
             {
                 t.y=S[0].y-2*SNAKE_D;
                 t.x=S[0].x;
                 for(i=len-1;i>0;i--)
                 S[i]=S[i-1];
                 S[i]=t;
                Draw_Snake(S,len);
                delay(50);               
             }}