排序的应用，地址： http://acm.pku.edu.cn/JudgeOnline/problem?id=1828
#include  " stdio.h "
int  num[ 50005 ][ 2 ];
 int  qu[ 50005 ];
 int  judge( int  a, int  b)
 {
    int ans=0;
    int i;
    for(i=0;i<2;i++)
    {
        if(num[qu[a]][i]>num[qu[b]][i]){ans=1;break;}
        if(num[qu[a]][i]<num[qu[b]][i]){ans=-1;break;}
    }
    return ans;
}
void  swap( int  a, int  b)
 {
    int t;
    t=qu[a];
    qu[a]=qu[b];
    qu[b]=t;
}
int  partion( int  low, int  high, int  p)
 {
    while(low<=high)
    {
        if(low<p)
        {
            if(judge(low,p)>0){swap(low,p);p=low;}
            low++;
        }
        else
        {
            if(high>p)
            {
                if(judge(high,p)<0){swap(high,p);p=high;}
            }
            high--;
        }
    }
    return p;
}
void  qsort( int  left, int  right)
 {
    int middle;
    if(left<right)
    {
        middle=(left+right)/2;
        middle=partion(left,right,middle);
        qsort(left,middle-1);
        qsort(middle+1,right);
    }
}
int  main()
 {
    int n,i;
    int count,last;
    while(1)
    {
        scanf("%d",&n);
        if(n==0)break;
        for(i=0;i<n;i++){scanf("%d%d",&num[i][0],&num[i][1]);qu[i]=i;}
        qsort(0,n-1);
        count=1;last=n-1;
        for(i=n-2;i>=0;i--)
        {
            if(num[qu[i]][0]!=num[qu[i+1]][0])
            {
                if(num[qu[i]][1]>num[qu[last]][1])
                {
                    count++;
                    last=i;
                }
            }
        }
        printf("%d\n",count);
    }
    return 0;
}