bzoj 3389: [Usaco2004 Dec]Cleaning Shifts安排值班 -- 贪心

#include<map>
#include<cmath>
#include<queue>
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
#define N 25010
struct qaz{int l,r;}a[N];
bool cmp(qaz a, qaz b){return a.l==b.l?a.r<b.r:a.l<b.l;}
int n,T,now,cnt,tmp,ans;
int main()
{
    scanf("%d%d",&n,&T);
    for(int i=1;i<=n;i++) scanf("%d%d",&a[i].l,&a[i].r);
    sort(a+1,a+n+1,cmp);
    while(now<T)
    {
        tmp=now;
        while(cnt<n&&a[cnt+1].l<=tmp+1){cnt++;now=max(now,a[cnt].r);}
        if((cnt==n&&a[cnt].r<T)||(a[cnt+1].l>now+1)){puts("-1");return 0;}
        ans++;
    }
    now<T?puts("-1"):printf("%d\n",ans);
}