贪心+优先队列+问题分解
对x，y 分开处理
当 xl<cnt（当前处理行）时，不能简单的选择cnt，而是应该让xl=cnt 并重新加入优先队列。（y的处理同上）
#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>
using namespace std; struct node {
int l,r;
int id;
friend bool operator < (const node &dis,const node &res){
if (dis.l!=res.l) return dis.l>res.l;
else return dis.r>res.r;
}
}rookx[],rooky[]; int ans[][];
int n; int solved (node* x,int p){
priority_queue<node> q;
while (!q.empty())
q.pop();
for (int i=;i<=n;i++)
q.push(x[i]);
int cnt=;
while (!q.empty() ){
node a;
a=q.top();
q.pop() ;
if (a.r<cnt)
return ;
if (a.l<cnt){
a.l=cnt;
q.push(a);
continue ;
}
if (a.l>cnt){
return ;
}
ans[p][a.id]=cnt++;
}
return ;
} int main (){
while (cin>>n&&n){
for (int i=;i<=n;i++){
cin>>rookx[i].l>>rooky[i].l>>rookx[i].r>>rooky[i].r;
rookx[i].id=rooky[i].id=i;
}
if (solved (rookx,)&&solved (rooky,)){
for (int i=;i<=n;i++)
cout<<ans[][i]<<" "<<ans[][i]<<endl;
}
else cout<<"IMPOSSIBLE"<<endl;
}
return ;
}