CQOIX2007余数之和

朴素能得个差不多吧……

这题改进算法真恶心

pascal一直过不了，难道非得转c++？

代码：（pascal）

var n,k,i,l,r,m:longint;
    ans:qword;
function ceil(x:real):longint;
 begin
 if trunc(x)<x then exit(trunc(x)+1) else exit(trunc(x));
 end;
procedure main;
 begin
   readln(n,k);
   ans:=0;
   if n>k then
    begin
     inc(ans,(n-k)*k);
     n:=k;
    end;
   m:=ceil(sqrt(k));
   for i:=1 to m do inc(ans,k mod i);
   for i:=1 to m do
    begin
      l:=(k div (i+1))+1;
      r:=(k div i);
      if l<=m then l:=m+1;
      if r>n then r:=n;
      if r<l then continue;
      inc(ans,(((k<<1)-i*(l+r))*(r-l+1)>>1));
    end;
   writeln(ans);
 end;
begin
  main;
end.    

代码：（c++）

#include <iostream>
#include <cmath>
using namespace std;

long long ans,n,k;
int main() {
  ios::sync_with_stdio(false);
  cin>>n>>k;
  if (n>k) {
    ans+=k*(n-k);
    n=k;
  }
  long long sqrtk=ceil(sqrt(k));
  for (long long i=1;i<=sqrtk;++i) ans+=k%i;
  for (long long a=1;a<=sqrtk;++a) {
    long long L=floor(k/(a+1))+1;
    long long R=floor(k/a);
    if (L<=sqrtk) L=sqrtk+1;
    if (R>n) R=n;
    if (R<L) continue;
    ans+=((k<<1)-a*L-a*R)*(R-L+1)>>1;
  }
  cout<<ans;
}

 另一种分块方法，pascal还是过不了……

#include <cmath>
#include <cstdio>
#include <algorithm>

long long n, k, ans;
int main()
{
  scanf("%lld%lld", &n, &k);
  if (n > k)
  {
    ans += (n-k)*k;
    n = k;
  }
  ans += n * k;
  for (long long i = 1, last; i <= n; i = last+1)
  {
    last = std::min(n, k/(k/i));
    ans -= (k/i) * (i+last) * (last-i+1) / 2;
  }
  printf("%lld", ans);
}