bzoj3157 3516

太神了，被数学题虐了

orz http://m.blog.csdn.net/blog/skywalkert/43970331

这道题关键是抓住m较小的特点，构造递推解决

 const mo=;

 var c:array[..,..] of longint;
     f:array[..] of int64;
     i,j,n,m:longint;
     t,ch:int64;

 function quick(x:int64;y:longint):int64;
   begin
     quick:=;
     while y> do
     begin
       if y mod = then quick:=quick*x mod mo;
       y:=y div ;
       x:=x*x mod mo;
     end;
   end;

 begin
   readln(n,m);
   c[,]:=;
   for i:= to m do
   begin
     c[i,]:=;
     for j:= to i do
       c[i,j]:=(c[i-,j]+c[i-,j-]) mod mo;
   end;
   if m= then
     writeln(int64(n)*int64(n+) div  mod mo)
   else begin
     ch:=quick(m-,mo-);
     f[]:=int64(m)*(quick(m,n)-+mo) mod mo*ch mod mo;
     for i:= to m do
     begin
       f[i]:=quick(n,i)*quick(m,n+) mod mo;
       for j:= to i- do
       begin
         t:=int64(c[i,j])*f[j] mod mo;
         if (i-j)  mod = then f[i]:=(f[i]-t+mo) mod mo
         else f[i]:=(f[i]+t) mod mo;
       end;
       f[i]:=f[i]*ch mod mo
     end;
     writeln(f[m]);
   end;
 end.