本人写的代码

a[i+j]=(x+y)%mod;
a[i+j+m]=((x-y)%mod+mod)%mod;

TLE

改成：

calc1(x,y) return x+y>=mod?x+y-mod:x+y;
calc2(x,y) return x-y<0?x-y+mod:x-y;
a[i+j]=calc1(x,y);
a[i+j+m]=calc2(x,y);

AC