区间覆盖问题，刘汝佳小白书P154页有具体思路;

代码例如以下：

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;

typedef struct
{
     int x,y;
}P;
P p[110000];

int cmp(P p1,P p2)
{
      if(p1.x==p2.x)
           return p1.y<p2.y;
      return p1.x<p2.x;
}

int main()
{
       int i,j,k,t,m,ans[110000],cnt;
       scanf("%d",&t);
       while(t--)
       {
              scanf("%d",&m);
              k=0;
              while(1)
              {
                     int x,y;
                     scanf("%d%d",&x,&y);
                     if(!x&&!y)
                          break;
                    p[k].x=x; p[k].y=y;
                    k++;
              }
              sort(p,p+k,cmp);
              cnt=0;
              int left=0,right,flag=0;
              for(i=0;i<k;)
              {
                   right=0;
                    while(p[i].x<=left&&i<k)
                   {
                            if(p[i].y>right)
                            {
                                   right=p[i].y;
                                   j=i;
                            }
                            i++;
                    }
                    if(right==0)
                        break;
                    left=right;
                    ans[cnt++]=j;
                    if(right>=m)
                    {
                         flag=1;
                         break;
                    }
              }
              if(!flag)
              {
                    printf("0\n");
                    if(t)
                        printf("\n");
                    continue;
              }
              printf("%d\n",cnt);
              for(i=0;i<cnt;i++)
                  printf("%d %d\n",p[ans[i]].x,p[ans[i]].y);
             if(t)
                 printf("\n");
       }
   return 0;
}