infomaniac's blog

Facebook HackerCup 2017 Round 1 Fighting Zombies Doubt
By infomaniac, history, 7 years ago, In English

I followed the official editorial for this and I seem to be getting WA on a handful of cases. I am using an optimzed version of the algorithm in the editorial. The logic is that for every zombie, we can construct 4 squares of side 'r' with the point at each of the square corners. So select any two points and for all 16 sets of square for these two, find the points in the union of these two squares and output the max. The exact code is given here

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7 years ago, # |
  Vote: I like it -10 Vote: I do not like it

Even i used the same approach don't know why i got WA 

//
// Created by RAHUL KUMAR SINGH on 15/01/17.
//

#include <cstdio>
#include <cmath>
#include <iostream>
#include <set>
#include <algorithm>
#include <vector>
#include <map>
#include <cassert>
#include <string>
#include <cstring>
#include <math.h>
#include <queue>
using  namespace std;

#define FOR(a,b) for(int i=a;i<=b;i++)
#define S(x) scanf("%d",&x)
#define S2(x,y) scanf("%d%d",&x,&y)
#define P(x) printf("%d\n",x)
#define all(v) v.begin(),v.end()
#define mp make_pair
#define pb push_back
#define FF first
#define SS second
#define PI 3.14159265
typedef long long int LL;
typedef int64_t i64;
int N;
LL R;
struct Rectangle{
    LL x;
    LL y;
    LL lenght;
};
inline bool operator<(const Rectangle& lhs, const Rectangle& rhs)
{
    return lhs.x!=rhs.y||lhs.y!=rhs.y||lhs.lenght!=rhs.lenght;
}

set<struct Rectangle> rectangles;
vector< pair <LL ,LL > > points;
struct Rectangle getupperRight(LL x,LL y,LL length){
    struct Rectangle temp;
    temp.x=x;
    temp.y=y;
    temp.lenght=length;
    return temp;
}

struct Rectangle getupperLeft(LL x,LL y,LL length){
    struct Rectangle temp;
    temp.x=x-length;
    temp.y=y;
    temp.lenght=length;
    return temp;
}

struct Rectangle getdownRight(LL x,LL y,LL length){
    struct Rectangle temp;
    temp.x=x;
    temp.y=y-length;
    temp.lenght=length;
    return temp;
}

struct Rectangle getdownLeft(LL x,LL y,LL length){
    struct Rectangle temp;
    temp.x=x-length;
    temp.y=y-length;
    temp.lenght=length;
    return temp;
}

void populateRectangles(int length){
    LL x,y;
    for(int i=0;i<points.size();i++){
        x=points[i].first;
        y=points[i].second;
        rectangles.insert(getupperRight(x,y,length));
        rectangles.insert(getupperLeft(x,y,length));
        rectangles.insert(getdownRight(x,y,length));
        rectangles.insert(getdownLeft(x,y,length));
    }
    return;
}
bool isInRectangle(struct Rectangle rectangle,pair<int,int> zombie){
    LL xlow,xhigh,ylow,yhigh;
    xlow=rectangle.x;
    ylow=rectangle.y;
    xhigh=rectangle.x+rectangle.lenght;
    yhigh=rectangle.y+rectangle.lenght;
    if(zombie.first<xlow || zombie.first>xhigh || zombie.second<ylow||zombie.second>yhigh)
        return false;
    return true;
}
int solve(){
    unsigned long ans=0;
    set< pair<int,int> > pointsInUnionRectangle;
    for(set<struct Rectangle>::iterator rectangle1=rectangles.begin();rectangle1!=rectangles.end();rectangle1++){
        for(set<struct Rectangle>::iterator rectangle2=rectangles.begin();rectangle2!=rectangles.end();rectangle2++){
            pointsInUnionRectangle.clear();
            for(int i=0;i<points.size();i++){
                if(isInRectangle(*rectangle1,points[i])){
                    pointsInUnionRectangle.insert(points[i]);
                }
                else if(isInRectangle(*rectangle2,points[i])){
                    pointsInUnionRectangle.insert(points[i]);
                }
            }
            ans=max(ans,pointsInUnionRectangle.size());
        }
    }
    return ans;
}
int main() {
    int tc;
    LL x, y;
    cin >> tc;
    for (int t = 1; t <= tc; t++) {
        points.clear();
        rectangles.clear();
        cin>>N>>R;
        for(int i=1;i<=N;i++) {
            cin >> x >> y;
            points.push_back(std::make_pair(x, y));
        }
        sort(points.begin(),points.end());
        populateRectangles(R);
        cout<<"Case #"<<t<<": "<<solve()<<endl;
    }
    return 0;
}
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    7 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    The issue is that we will miss cases like Bassel suggested. Hence you need two points to decide a square with one point on top edge and one on the left edge.

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      7 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      infomaniac we can define the square with one lower left corner and side length ?

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        7 years ago, # ^ |
        ← Rev. 3   Vote: I like it 0 Vote: I do not like it

        Yes you can technically define a square like this but you cannot guarantee that an optimal square has a point on a corner, that's the issue.

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          7 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          So how can i get that all possible squares can you put some light on it ?

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            7 years ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            As already stated, consider a pair of points for a square, with one on the top edge and one on the left edge. Refer to the official editorial for details.

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7 years ago, # |
← Rev. 3   Vote: I like it +11 Vote: I do not like it

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Your algorithm will fail a test case like this one based on your description

It will print 6 instead of 8

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