BZOJ 3156: 防御准备 斜率优化DP

#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <queue>
using namespace std;
const int N = 1e6+20,inf = 2e9, mod = 1e9+7;
typedef long long ll;

ll dp[N];
ll n,a[N],b[N];
double getsum(ll k,ll j)
{
    return (double)((dp[k]-dp[j]) + (double)(k*k+k-j*j-j)/2.0)/(double)(k-j);
}
int main()
{
    scanf("%lld",&n);
    for(int i=1;i<=n;i++)
    {
        scanf("%lld",&b[i]);
    }
    for(int i=1;i<=n;i++)
    {
        a[i]=b[n-i+1];
    }
    deque<int > q;
    dp[1] = a[1];
    q.push_back(1);
    for(int i=2;i<=n+1;i++)
    {
        int now=q.front();q.pop_front();
        while(!q.empty()&&getsum(q.front(),now)<i) now=q.front(),q.pop_front();
        q.push_front(now);
        dp[i] = dp[now] + (ll)(i-now-1)*(ll)(i-now)/2 + a[i];
        now = q.back();q.pop_back();
        while(!q.empty()&&getsum(i,now)<getsum(now,q.back())) now=q.back(),q.pop_back();
        q.push_back(now);
        q.push_back(i);
    }
    printf("%lld\n",dp[n+1]);

    return 0;
}