AC日记——松江1843路 洛谷七月月赛

松江1843路 

思路：

　　三分；

代码：

#include <bits/stdc++.h>
using namespace std;
#define maxn 100005
#define ll long long
struct DataType
{
    ll pi,val,sumval,sumpi;
    bool operator<(const DataType pos)const
    {
        return pi<pos.pi;
    }
};
struct DataType ai[maxn];
ll len,n;
inline void in(ll &now)
{
    char Cget=getchar();now=0;
    while(Cget>'9'||Cget<'0')Cget=getchar();
    while(Cget>='0'&&Cget<='9')
    {
        now=now*10+Cget-'0';
        Cget=getchar();
    }
}
ll lower(ll key)
{
    ll l=1,r=n,res=1,mid;
    while(l<=r)
    {
        mid=l+r>>1;
        if(key<=ai[mid].pi) res=mid,r=mid-1;
        else l=mid+1;
    }
    return res;
}
ll calculate(ll key)
{
    ll now=lower(key);
    ll res1=key*ai[now-1].sumval-ai[now-1].sumpi;
    ll res2=ai[n].sumpi-ai[now-1].sumpi-key*(ai[n].sumval-ai[now-1].sumval);
    return res2+res1;
}
int main()
{
    in(len),in(n);
    for(ll i=1;i<=n;i++) in(ai[i].pi),in(ai[i].val);
    sort(ai+1,ai+n+1);
    for(ll i=1;i<=n+1;i++)
    {
        ai[i].sumval=ai[i-1].sumval+ai[i].val;
        ai[i].sumpi=ai[i-1].sumpi+ai[i].val*ai[i].pi;
    }
    ll l=0,r=len,mid,now,nowl,nowr;
    while(l<r)
    {
        mid=l+r>>1;
        now=calculate(mid);
        nowl=calculate(mid-1);
        nowr=calculate(mid+1);
        if(nowl<now) r=mid-1;
        else if(nowr<now) l=mid+1;
        else
        {
            printf("%lld
",calculate(mid));
            return 0;
        }
    }
    printf("%lld
",calculate(l));
    return 0;
}