文件读入写出[noi格式]

freopen("a.in","r",stdin);
freopen("a.out","w",stdout);

文件读入写出其他用法

freopen("a.a","r",stdin);
freopen("a.b","w",stdout);

freopen("a.v","r",stdin);
freopen("a.d","w",stdout);

freopen("a.num","r",stdin);
freopen("a.test","w",stdout);