【bzoj 3156】防御准备

 1 #include<cstdio>
 2 #include<algorithm>
 3 #include<cstring>
 4 using namespace std;
 5 const int N=1000050;
 6 long long a[N],f[N],n,t,q[N],l=0,r=0;
 7 long long read()
 8 {
 9     long long x=0,f=1;char c=getchar();
10     while(c<‘0‘||c>‘9‘){if(c==‘-‘)f=-1;c=getchar();}
11     while(c>=‘0‘&&c<=‘9‘){x=x*10+c-‘0‘;c=getchar();}
12     return x*f;
13 }
14 double slope(long long k,long long j)
15 {return (2.0*(f[j]-f[k])+j*(j+1)-k*(k+1))/(2.0*(j-k));}
16 int main()
17 {
18     n=read();
19     for(int i=1;i<=n;i++)a[i]=read();
20     for(long long i=1;i<=n;i++)
21     {
22         while(l<r&&slope(q[l],q[l+1])<i)l++;
23         t=q[l];
24         f[i]=f[t]+(long long)(i-t)*(i-t-1)/2+a[i];
25         while(l<r&&slope(q[r-1],q[r])>slope(q[r],i))r--;
26         q[++r]=i;
27     }
28     printf("%lld",f[n]);
29     return 0;
30 }
31