【NOIP2016练习】T1 string （计数）

题意：

思路：

 const mo=;
 var pow,f,exf:array[-..]of int64;
     n,k,i:longint;
     ans,x,y:int64;
     s,t:ansistring;

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

 function min(x,y:int64):int64;
 begin
  if x<y then exit(x);
  exit(y);
 end;

 begin
  assign(input,'1.in'); reset(input);
  assign(output,'1.out'); rewrite(output);
  readln(n,k);
  readln(s);
  readln(t);
  pow[]:=;
  for i:= to n do pow[i]:=pow[i-]* mod mo;
  f[]:=;
  for i:= to n do f[i]:=f[i-]*i mod mo;
  exf[]:=; exf[]:=;
  for i:= to n do exf[i]:=exf[mo mod i]*(mo-mo div i) mod mo;
  for i:= to n do exf[i]:=exf[i-]*exf[i] mod mo;
  ans:=;
  for i:= to n do
  begin
   x:=ord(t[i])-ord('a'); y:=ord(s[i])-ord('a');
   ans:=(ans+min(x,y)*c(n-i,k-) mod mo*pow[k-] mod mo) mod mo;
   if s[i]<t[i] then
   begin
    ans:=(ans+c(n-i,k)*pow[k] mod mo) mod mo;
    ans:=(ans+(x-y-)*c(n-i,k-) mod mo*pow[k-] mod mo) mod mo;
   end;
   if s[i]<>t[i] then dec(k);
  end;
  writeln(ans);
  close(input);
  close(output);
 end.