ANTIPODAL-Editorial

editorial
, ,
21

Hey why are you checking the second angle in different way

                double m1 = atan(1.0 * (y[i] - y[j]) / (x[i] - x[j])) * 180 / PI;
                double m2 = atan(1.0 * (x[j] - x[i]) / (y[i] - y[j])) * 180 / PI;

The first angle i.e m1 is angle with X axis and what about second one i.e m2 ??

22

@pratik_nextrix @shivam_2701

Here m1 is the current slope between ith and jth point . m[m1]++ implies i am incrementing the count of this slope .
m2 is the slope perpendicular to m1 . ans+=m[m2] implies that i am adding the count of points which have slope as m2 . Because they will form right angle triangle
Try to do dry run for better understanding .

23

Nice solution .

1 Like
24

@adithya5768 Hey I have changed this code to use a vector now can you please help me i am getting a runtime error
https://www.codechef.com/viewsolution/63692584
Here is the link of the submission

25

@shivam_2701 Are you aware that your code could use up to O(n^3) space?
For each line segment, you are creating a vector of size n. There could be n^2 line segments.

There probably is so much unnecessary information being stored in memory.
I do not know if that is the limit for memory usage in this platform.

Besides, like I mentioned before, you do NOT need to use a “global” map. Just declare a map inside the first for loop and use it inside the second for loop.

27

Thank you brother I was able to correct my code and now it passes all he test case
my final code
https://www.codechef.com/viewsolution/63730641

1 Like
28

For this data, I test several diffrent AC code(include the author and the tester), the answer are diffrent, who are corrected？

1
10
0 0 
0 1
1 0
1 1
2 2
1 2
2 1
3 3
2 3
3 2
29

Hey @huas_huah :wave: ,
For your test case Testers 2 Solution is giving wrong answer reason is it is not counting the numerator and denominator pairs that are 0. You can go through the modified code to understand properly.

#include <bits/stdc++.h>
using namespace std;
#ifndef ONLINE_JUDGE    
#define debug(x) cerr<<#x<<" "; _print(x); cerr<<nline;
#else
#define debug(x);  
#endif 
#define ll int
 
 
/*
------------------------Input Checker----------------------------------
*/
 
long long readInt(long long l,long long r,char endd){
    long long x=0;
    int cnt=0;
    int fi=-1;
    bool is_neg=false;
    while(true){
        char g=getchar();
        if(g=='-'){  
            assert(fi==-1);
            is_neg=true;
            continue;
        }
        if('0'<=g && g<='9'){
            x*=10;
            x+=g-'0';
            if(cnt==0){
                fi=g-'0';
            }
            cnt++;
            assert(fi!=0 || cnt==1);
            assert(fi!=0 || is_neg==false);
 
            assert(!(cnt>19 || ( cnt==19 && fi>1) ));
        } else if(g==endd){
            if(is_neg){
                x= -x;
            }
 
            if(!(l <= x && x <= r))
            {
                cerr << l << ' ' << r << ' ' << x << '\n';
                assert(1 == 0);
            }
 
            return x;
        } else {
            assert(false);
        }
    }
}
string readString(int l,int r,char endd){
    string ret="";
    int cnt=0;
    while(true){
        char g=getchar();
        assert(g!=-1);
        if(g==endd){
            break;
        }
        cnt++;
        ret+=g;
    }
    assert(l<=cnt && cnt<=r);
    return ret;
}
long long readIntSp(long long l,long long r){
    return readInt(l,r,' ');
}
long long readIntLn(long long l,long long r){
    return readInt(l,r,'\n');
}
string readStringLn(int l,int r){
    return readString(l,r,'\n');
}
string readStringSp(int l,int r){
    return readString(l,r,' ');
}
 
 
/*
------------------------Main code starts here----------------------------------
*/
ll MAX=1e9;
ll tes_sum=0;
void solve(){  
    ll n;
    cin>>n;
    tes_sum+=n;
    vector<ll> x(n+5),y(n+5);
    ll ans=0;
    set<pair<ll,ll>> check;
    for(ll i=1;i<=n;i++){
        cin>>x[i]>>y[i];
        check.insert({x[i],y[i]}); 
    }
    assert(check.size()==n); 
    for(ll i=1;i<=n;i++){
        map<pair<ll,ll>,ll> freq; 
        int X = 0,Y = 0;
        for(ll j=1;j<=n;j++){
            if(i==j){
                continue;
            }
            ll num=y[i]-y[j],denom=x[i]-x[j];
            ll g=__gcd(num,denom);
            num/=g; denom/=g;
            if(denom<0){
                denom=-denom,num=-num;
            }
            if(num == 0) X++;
            if(denom == 0) Y++;
            if(!num || !denom) continue;
            freq[{num,denom}]++;
            ll l=-denom,r=num;
            if(r<0){
                r=-r,l=-l;
            }
            ans+=freq[{l,r}]; 
        }
        ans += X*Y;
    }
    cout<<ans<<"\n"; 
    return;
}
int main(){
    ios_base::sync_with_stdio(false);                         
    cin.tie(NULL);                              
    #ifndef ONLINE_JUDGE                 
    freopen("input.txt", "r", stdin);                                              
    freopen("output.txt", "w", stdout);  
    freopen("error.txt", "w", stderr);                          
    #endif         
    ll test_cases;
    cin>>test_cases;
    while(test_cases--){
        solve();
    }
    assert(getchar()==-1);
    assert(tes_sum<=2000);
    return 0;
}
30

So, the test cases were weak, right?

31

Yes.