2018牛客多校第二场 G transform（二分）

题目链接：https://www.nowcoder.com/acm/contest/140/G

这位博主的博客写的很详细

#include<bits/stdc++.h>
using namespace std;
#define ll long long
const int N=1e6+10;
struct node
{
    ll x,w;
}a[N];
using namespace std;
ll n,T;
ll prew[N],prec[N],sufw[N],sufc[N];
ll cal_pre(ll l,ll r)
{
    return prec[r]-prec[l-1]-prew[l-1]*(a[r].x-a[l-1].x);
}
ll cal_suf(ll l,ll r)
{
    return sufc[l]-sufc[r+1]-sufw[r+1]*(a[r+1].x-a[l].x);
}
bool check(ll num)
{
    ll num2=num/2+1;
    ll l=1,r=1,mid=1;
    while(1)
    {
        while(r<=n&&prew[r]-prew[l-1]<num)r++;
        while(mid<=n&&prew[mid]-prew[l-1]<num2)mid++;
        if(r>n||mid>n)break;
        ll s=cal_pre(l,mid)+cal_suf(mid,r-1)+(num-(prew[r-1]-prew[l-1]))*(a[r].x-a[mid].x);
        if(s<=T)return true;
        l++;
    }
    l=r=mid=n;
    while(1)
    {
        while(l>=1&&prew[r]-prew[l-1]<num)l--;
        while(mid>=2&&prew[mid]-prew[l-1]<num2)mid--;
        if(l<1||mid<2)break;
        ll s=cal_pre(l+1,mid)+cal_suf(mid,r)+(num-(prew[r]-prew[l]))*(a[mid].x-a[l].x);
        if(s<=T)return true;
        r--;
    }
    return false;
}
int main()
{
    scanf("%lld%lld",&n,&T);
    T/=2;
    ll l=0,r=0;
    for(ll i=1;i<=n;i++)
    {
        scanf("%lld",&a[i].x);
    }
    for(ll i=1;i<=n;i++)
    {
        scanf("%lld",&a[i].w);
    }
    for(ll i=1;i<=n;i++)
    {
        prew[i]=prew[i-1]+a[i].w;
        prec[i]=prec[i-1]+prew[i-1]*(a[i].x-a[i-1].x);
    }
    for(ll i=n;i>=1;i--)
    {
        sufw[i]=sufw[i+1]+a[i].w;
        sufc[i]=sufc[i+1]+sufw[i+1]*(a[i+1].x-a[i].x);
        r+=a[i].w;
    }
    while(l<r)
    {
        ll mid=(l+r+1)>>1;
        if(check(mid))
        {
            l=mid;
        }
        else r=mid-1;
    }
    printf("%lld\n",l);
    return 0;
}