bzoj 3156: 防御准备

#include<cstdio>
#include<iostream>
#define N 1000001
using namespace std;
long long sum[N],dp[N];
int a[N];
int head,tail,q[N];
void read(int &x)
{
    x=0; char c=getchar();
    while(!isdigit(c))  c=getchar();
    while(isdigit(c)) { x=x*10+c-‘0‘; c=getchar(); }
}
long long up(int k,int j)
{
    return dp[j]-dp[k]+sum[j]-sum[k];
}
long long down(int k,int j)
{
    return j-k;
}
int main()
{
    int n;
    read(n);
    for(int i=1;i<=n;i++) read(a[i]),sum[i]=sum[i-1]+i;
    int j;
    for(int i=1;i<=n;i++)
    {
        while(head<tail && up(q[head],q[head+1])<=down(q[head],q[head+1])*i) head++;
        j=q[head];
        dp[i]=dp[j]+1ll*i*(i-j)-sum[i]+sum[j]+a[i];
        while(head<tail && up(q[tail-1],q[tail])*down(q[tail],i)>=up(q[tail],i)*down(q[tail-1],q[tail])) tail--;
        q[++tail]=i;
    }
    printf("%lld",dp[n]);
}