BZOJ 4723 Flappy Bird

找到可行区间，最优解一定在区间的下端点。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxn 500500
using namespace std;
int n,X,x[maxn],a[maxn],b[maxn],l[maxn],r[maxn],ans=;
int read()
{
    char ch;int data=,f=;
    while (ch<'' || ch>'') {if (ch=='-') f=-;ch=getchar();}
    while (ch>='' && ch<='')
    {
        data=data*+ch-'';
        ch=getchar();
    }
    return data*f;
}
int main()
{
    n=read();X=read();
    for (int i=;i<=n;i++)
        x[i]=read(),a[i]=read(),b[i]=read();
    l[]=r[]=;
    for (int i=;i<=n;i++)
    {
        l[i]=max(l[i-]-(x[i]-x[i-]),a[i]+);
        r[i]=min(r[i-]+(x[i]-x[i-]),b[i]-);
        if (l[i]>r[i]) {printf("NIE\n");return ;}
    }
    int ret=;
    for (int i=;i<=n;i++)
    {
        if (x[i]>X) break;
        int now=x[i]-x[i-]+l[i]-ret;
        if (now%) {l[i]++;now++;}
        if (l[i]>r[i]) {printf("NIE\n");return ;}
        if (now>) ans+=now/;ret=l[i];
    }
    printf("%d\n",ans);
    return ;
}