BZOJ3126: [Usaco2013 Open]Photo

n<=200000个点，m<=100000个区间，每个区间有且仅有一个点，求最多几个点，无解-1。

http://www.cnblogs.com/Chorolop/p/7570191.html

WA了两次：看成最少几个点；判无解是<0而不一定=-inf。

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<algorithm>
//#include<iostream>
using namespace std;

int n,m;
#define maxn 200011
int l[maxn],r[maxn],f[maxn];
int que[maxn],head,tail;
int x,y;const int inf=0x3f3f3f3f;
int main()
{
    scanf("%d%d",&n,&m);
    for (int i=1;i<=n+1;i++) r[i]=i-1;
    memset(l,0,sizeof(l));
    for (int i=1;i<=m;i++)
    {
        scanf("%d%d",&x,&y);
        r[y]=min(r[y],x-1);
        l[y+1]=max(l[y+1],x);
    }
    for (int i=n;i>=1;i--) r[i]=min(r[i+1],r[i]);
    for (int i=2;i<=n+1;i++) l[i]=max(l[i-1],l[i]);
    que[head=(tail=1)-1]=0;f[0]=0;
    for (int i=1;i<=n+1;i++)
    {
        for (int j=r[i-1]+1;j<=r[i];j++)
        {
            while (head<tail && f[que[tail-1]]<=f[j]) tail--;
            que[tail++]=j;
        }
        while (head<tail && que[head]<l[i]) head++;
        if (head<tail) f[i]=f[que[head]]+1;
        else f[i]=-inf;
    }
    if (f[n+1]<0) puts("-1");
    else printf("%d\n",f[n+1]-1);
    return 0;
}