小A与最大子段和 斜率优化 + 二分 + 细节

Code:

#include <bits/stdc++.h>
#define setIO(s) freopen(s".in","r",stdin)
#define x(i) (1.0000*i)
#define y(i) (1.0000*s2[i]*i-1.00000*s1[i])
#define maxn 300000
#define ll long long
#define ldb double
using namespace std;
int tail,head,n;
int q[maxn];
ll s1[maxn],s2[maxn],f[maxn];
ldb slope(int i,int j) { return (ldb)(1.0*y(i)-y(j))/(ldb)(1.0*x(i)-x(j));    }
int main()
{
	int i,j;
	ll ans=-1e18;
	// setIO("input");
	scanf("%d",&n);
	for(i=1;i<=n;++i)
	{
		scanf("%lld",&s2[i]);
		s1[i]=s1[i-1]+s2[i]*i;
		s2[i]+=s2[i-1];
	}
	head=tail=1;
	for(i=1;i<=n;++i)
	{
		int l=head,r=tail,re=0,mid=0;
		while(l<=r)
		{
			mid=(l+r)>>1;
			if(mid==tail) { re=tail;  break;    }
			if(slope(q[mid],q[mid+1])<=s2[i]) re=mid,r=mid-1;
			else l=mid+1;
		}
		f[i]=s1[i]-s1[q[re]]-(s2[i]-s2[q[re]])*q[re];
		ans=max(ans,f[i]);
		while(head<tail&&slope(i,q[head-1])>slope(q[head],q[head-1])) --tail;
		q[++tail]=i;
	}
	printf("%lld\n",ans);
	return 0;
}