【BZOJ4517】排列计数（排列组合）

题意：1-n的一个序列，其中有m个a[i]=i，求方案数

n,m<=1000000

题意：显然ANS=c(n,m)*d[n-m]

d[i]为错排方案数=d[i-1]*n+(-1)^n

const mo=1000000007;
var fac,exf,d:array[0..1000000]of int64;
    cas,i,n,m:longint;

function c(x,y:longint):int64;
begin
 exit(fac[x]*exf[y] mod mo*exf[x-y] mod mo);
end;

begin
 assign(input,'bzoj4517.in'); reset(input);
 assign(output,'bzoj4517.out'); rewrite(output);
 d[1]:=0;
 for i:=2 to 1000000 do
 begin
  d[i]:=d[i-1]*i;
  if i and 1=0 then inc(d[i])
   else dec(d[i]);
  d[i]:=d[i] mod mo;
 end;
 fac[0]:=1; exf[0]:=1; exf[1]:=1;
 for i:=1 to 1000000 do fac[i]:=fac[i-1]*i mod mo;
 for i:=2 to 1000000 do exf[i]:=exf[mo mod i]*(mo-mo div i) mod mo;
 for i:=1 to 1000000 do exf[i]:=exf[i-1]*exf[i] mod mo;
 readln(cas); d[0]:=1;
 for i:=1 to cas do
 begin
  readln(n,m);
  writeln(c(n,m)*d[n-m] mod mo);
 end;
 close(input);
 close(output);
end.