uva 10763 Foreign Exchange

#include<stdio.h>#include<string.h>int main(){    int map[1024][1024],a,b;    int n,i,j;    bool f;    while(~scanf("%d",&n))    {        if(n==0) break;        memset(map,0,sizeof(map));        f=0;        for(i=0;i<n;i++)        {            scanf("%d%d",&a,&b);            map[a][b]++;        }        for(i=0;i<1024;i++)            {                for(j=0;j<1024;j++)                if(map[i][j]!=map[j][i]) {f=1;break;}                if(f==1) break;            }        if(f) printf("NO\n");        else printf("YES\n");    }    return 0;}