POJ 2785 4 Values whose Sum is 0

you can find the problem here
the question let us to find 4 number from each row which sum fo them is zero. what we should do is to take two rows of tem as one row,get 2∗N∗N number, than us binary search to find compatible solution .

#include<iostream>#include <cstdio>#include<cstring>#include <algorithm>using namespace std;const int maxn = 4005;int a[maxn],b[maxn],c[maxn],d[maxn];int ab[maxn*maxn],cd[maxn*maxn];int main(){    int n = 0;    while(~ scanf("%d",&n))    {        for(int i=1; i<=n; i++) scanf("%d%d%d%d",&a[i],&b[i],&c[i],&d[i]);        int cnt = 0;        for(int i=1; i<=n; i++)        {            for(int j = 1; j<=n; j++)            {                ab[cnt] = a[i]+b[j];                cd[cnt++] = c[i]+d[j];            }        }        int ans = 0;        //sort(ab,ab+cnt);        sort(cd,cd+cnt);        for(int i=0; i<cnt; i++)        {            int s = ab[i];            //cout<<ab[i]<<endl;            if(binary_search(cd,cd+cnt,-s))            {                ans+=upper_bound(cd,cd+cnt,-s)-lower_bound(cd,cd+cnt,-s);            }        }        printf("%d\n",ans);    }    return 0;}