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右端点升序，取右端点

暴力删边

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <iostream>
#include <map>
#include <set>
#include <math.h>
using namespace std;
#define inf 10000000
#define ll int
#define N 2105
#define L(x) (x<<1)
#define R(x) (x<<1|1)
inline ll Mid(ll a,ll b){return (a+b)>>1;}
ll n;
struct node{
	ll x,y;
}a[N];
bool cmp(node a,node b){
	if(a.y==b.y)return a.x<b.x;
	return a.y<b.y;
}
vector<ll>ans;
ll h[N];
int main(){
	ll i, j;
	while(~scanf("%d",&n)){
		for(i=1;i<=n;i++){
			cin>>a[i].x>>a[i].y;
			if(a[i].x>a[i].y)swap(a[i].x,a[i].y);
		}
		memset(h, 0, sizeof h);
		sort(a+1,a+n+1,cmp);
		ans.clear();
		for(i = 1; i <=n;i++)if(!h[i]){
			ans.push_back(a[i].y);
			for(j=i;j<=n;j++)
			{
				if(a[j].x<=a[i].y && a[i].y<=a[j].y)
					h[j] = 1;
			}
		}
		cout<<ans.size()<<endl;
		for(i=0;i<ans.size();i++)
			printf("%d%c",ans[i],i==((ll)ans.size()-1)?'\n':' ');
	}
	return 0;
}
/*
3
40 -83
52 -80
-21 -4

*/
/*
struct node{
ll l, r, id;
ll maxx,val,lmaxx;
}tree[N];
void push_down(ll id){
if(tree[id].l==tree[id].r)return ;
if(tree[id].val){
tree[L(id)].val += tree[id].val;
tree[R(id)].val += tree[id].val;
tree[L(id)].lmaxx += tree[id].val;
tree[L(id)].maxx += tree[id].val;
tree[R(id)].lmaxx += tree[id].val;
tree[R(id)].maxx += tree[id].val;
tree[id].val = 0;
}
}
void build(ll l, ll r, ll id){
tree[id].l = l, tree[id].r = r;
tree[id].maxx = tree[id].val = 0;
tree[id].lmaxx = 0;
if(l==r)return ;
ll mid = Mid(l,r);
build(l,mid,L(id)); build(mid+1,r,R(id));
}
void updata(ll l, ll r, ll val, ll id){
push_down(id);
if(l == tree[id].l && tree[id].r == r){
tree[id].val += val;
tree[id].maxx += val;
tree[id].lmaxx += val;

return;
}
ll mid = Mid(tree[id].l, tree[id].r);
if(mid<l)updata(l,r,val,R(id));
else if(r<=mid)
updata(l,r,val,L(id));
else {
updata(l,mid,val,L(id));
updata(mid+1,r,val,R(id));
}
tree[id].maxx = max(tree[L(id)].maxx, tree[R(id)].maxx);
tree[id].lmaxx = tree[L(id)].lmaxx;
}
struct hehe{
ll val, x;
hehe(ll a=0,ll b=0):val(a),x(b){}
bool operator<(const hehe&a)const{
if(a.val==val)return a.x<x;
return a.val>val;
}
};
hehe query(ll l, ll r, ll id){
push_down(id);
if(l == tree[id].l && tree[id].r == r && tree[id].lmaxx==tree[id].maxx)
return hehe(tree[id].maxx, tree[id].l);
ll mid = Mid(tree[id].l, tree[id].r);
if(mid<l)return query(l,r,R(id));
else if(r<=mid)return query(l,r,L(id));
else return max(query(l,mid,L(id)),query(mid+1,r,R(id)));
}
set<ll>myset;
set<ll>::iterator p;
map<ll,ll>mp;
ll pos[N]; 
vector<ll>ans;
int use[N];
int main(){
ll i, j, u, v;
while(cin>>n){
myset.clear();
mp.clear();
ans.clear();
for(i=1;i<=n;i++){
cin>>x[i]>>y[i];
if(x[i]>y[i])swap(x[i],y[i]);
myset.insert(x[i]);
myset.insert(y[i]);
use[i] = true;
}
ll siz = 1;
for(p=myset.begin(); p!=myset.end(); p++, siz++) {
pos[siz] = *p;
mp.insert(pair<ll,ll>(*p,siz));
}
for(i=1;i<=n;i++) {
x[i] = mp.find(x[i])->second;
y[i] = mp.find(y[i])->second;
}
build(1,siz-1,1);
for(i=1;i<=n;i++)
updata(x[i],y[i],1,1);
while(tree[1].maxx){
hehe now = query(1,siz-1,1);
ans.push_back(pos[now.x]);
for(i=1;i<=n;i++)
if(x[i]<=now.x&&now.x<=y[i] && use[i])
updata(x[i],y[i],-1,1), use[i] = false;
}
cout<<ans.size()<<endl;
for(i=0;i<ans.size();i++)
printf("%d%c",ans[i],i==((ll)ans.size()-1)?'\n':' ');
}
return 0;
}
/**/