斜率优化实现初步（4） [BZOJ][1911][Apio2010]特别行动队

#include<bits/stdc++.h>
using namespace std;

#define int long long
const int MAXN=1e6+2333;
int n,a,b,c;
int x[MAXN],sum[MAXN];
int f[MAXN],g[MAXN];
int head=1,tail=1,q[MAXN];

char readc(){
    static char buf[100000], *l=buf,*r=buf;
    if (l==r) r=(l=buf)+fread(buf,1,100000,stdin);
    if (l==r) return EOF;else return *l++;
}

int read(){
    int x=0,f=1;char ch=readc();
    while (!isdigit(ch)) { if (ch=='-') f=-1; ch=readc();}
    while (isdigit(ch)) x=x*10+ch-48,ch=readc();
    return x*f;
}

int slope(int j,int k){
    return (g[j]-g[k])/(sum[j]-sum[k]);
}

int pow2(int x){
    return x*x;
}

signed main(){
    n=read();
    a=read(),b=read(),c=read();
    for (int i=1;i<=n;i++)
        x[i]=read(),sum[i]=sum[i-1]+x[i];
    for (int i=1;i<=n;i++){
        while (head<tail&&slope(q[head+1],q[head])>2*a*sum[i]+b)
            head++;
        f[i]=f[q[head]]+a*pow2(sum[i]-sum[q[head]])+b*(sum[i]-sum[q[head]])+c;
        g[i]=f[i]+a*pow2(sum[i]);
        while (head<tail&&slope(i,q[tail])>slope(q[tail],q[tail-1]))
            tail--;
        q[++tail]=i;
    }
    printf("%lld\n",f[n]);
    return 0;
}