60pts求助

#include<cstdio>
#include<algorithm>
using namespace std;
#define ll long long 

const ll N=3005;
ll n,m;
ll a1[N],b1[N];
ll val[N];
ll a[N],b[N];
ll ans;

struct node{
    ll l,r,tag,num;
}Tree[N<<2];

void build(ll l,ll r,ll rt){
    Tree[rt].l=l,Tree[rt].r=r;
    if(l==r)return ;
    ll m=(l+r)/2;
    build(l,m,rt<<1),build(m+1,r,rt<<1|1);
}

void pushdown(ll rt){
    if(Tree[rt].num){
        Tree[rt<<1].num=Tree[rt].num;
        Tree[rt<<1|1].num=Tree[rt].num;
        Tree[rt].num=0;
    }
    return ;
}

void update(ll rt,ll L,ll R,ll val){
    ll l=Tree[rt].l,r=Tree[rt].r;
    if(r<L||l>R)return ;
    if(L<=l&&r<=R){
        Tree[rt].num=val;
        return ;
    }
    pushdown(rt);
    update(rt<<1,L,R,val);
    update(rt<<1|1,L,R,val);
    return ;
}

ll cnt[N<<1];

void pushit(ll rt){
    if(Tree[rt].l==Tree[rt].r)return ;
    pushdown(rt);
    pushit(rt<<1),pushit(rt<<1|1);
}

void findit(ll rt){
    if(Tree[rt].l==Tree[rt].r){
        cnt[Tree[rt].l]=Tree[rt].num;
        return ;
    }
    pushdown(rt);
    findit(rt<<1),findit(rt<<1|1);
}

bool tmp[N<<2];
signed main(){
    scanf("%lld%lld",&n,&m);
    for(register ll i=1;i<=m;++i){
        scanf("%lld%lld",&a1[i],&b1[i]);
        val[i*2-1]=a1[i];
        val[i*2]=b1[i];
    }
    val[2*m+1]=n;
    sort(val+1,val+1+m*2+1);
    ll tot=unique(val+1,val+1+2*m+1)-val-1;
    for(register ll i=1;i<=m;++i){
        ll pos1,pos2;
        pos1=lower_bound(val+1,val+1+tot,a1[i])-val;
        pos2=lower_bound(val+1,val+1+tot,b1[i])-val;
        a[i]=pos1;
        b[i]=pos2;
    }
    n=lower_bound(val+1,val+1+tot,n)-val;
    build(1,n,1);
    for(register ll i=1;i<=m;++i)
        update(1,a[i],b[i],i);
    for(register ll i=1;i<=m;++i)pushit(1);
    findit(1);
    for(register ll i=1;i<=n;++i)
        if(!tmp[cnt[i]] && cnt[i])
            ans++,tmp[cnt[i]]=1;
    printf("%lld\n",ans);
    return 0;
}