[Solution] Two Houses Codeforces Round #711

Two Houses Solution Codeforces

This is an interactive problem. Remember to flush your output while communicating with the testing program. You may use fflush(stdout) in C++, system.out.flush() in Java, stdout.flush() in Python or flush(output) in Pascal to flush the output. If you use some other programming language, consult its documentation. You may also refer to the guide on interactive problems: https://codeforces.com/blog/entry/45307.

There is a city in which Dixit lives. In the city, there are nn houses. There is exactly one directed road between every pair of houses. For example, consider two houses A and B, then there is a directed road either from A to B or from B to A but not both. The number of roads leading to the ii-th house is kiki.

Two houses A and B are bi-reachable if A is reachable from B and B is reachable from A. We say that house B is reachable from house A when there is a path from house A to house B.

Dixit wants to buy two houses in the city, that is, one for living and one for studying. Of course, he would like to travel from one house to another. So, he wants to find a pair of bi-reachable houses A and B. Among all such pairs, he wants to choose one with the maximum value of |kA−kB||kA−kB|, where kiki is the number of roads leading to the house ii. If more than one optimal pair exists, any of them is suitable.

Since Dixit is busy preparing CodeCraft, can you help him find the desired pair of houses, or tell him that no such houses exist?

In the problem input, you are not given the direction of each road. You are given — for each house — only the number of incoming roads to that house (kiki).

You are allowed to ask only one type of query from the judge: give two houses A and B, and the judge answers whether B is reachable from A. There is no upper limit on the number of queries. But, you cannot ask more queries after the judge answers “Yes” to any of your queries. Also, you cannot ask the same query twice.

Once you have exhausted all your queries (or the judge responds “Yes” to any of your queries), your program must output its guess for the two houses and quit.

See the Interaction section below for more details.


The first line contains a single integer nn (3≤n≤5003≤n≤500) denoting the number of houses in the city. The next line contains nn space-separated integers k1,k2,…,knk1,k2,…,kn (0≤ki≤n−10≤ki≤n−1), the ii-th of them represents the number of incoming roads to the ii-th house.Interaction

To ask a query, print “? A B” (1≤A,B≤N,A≠B)(1≤A,B≤N,A≠B). The judge will respond “Yes” if house B is reachable from house A, or “No” otherwise.

To output the final answer, print “! A B”, where A and B are bi-reachable with the maximum possible value of |kA−kB||kA−kB|. If there does not exist such pair of houses A and B, output “! 0 0”.

After outputting the final answer, your program must terminate immediately, otherwise you will receive Wrong Answer verdict.

You cannot ask the same query twice. There is no upper limit to the number of queries you ask, but, you cannot ask more queries after the judge answers “Yes” to any of your queries. Your program must now output the final answer (“! A B” or “! 0 0”) and terminate.

If you ask a query in incorrect format or repeat a previous query, you will get Wrong Answer verdict.

After printing a query do not forget to output the end of the line and flush the output. Otherwise, you will get the Idleness limit exceeded error. To do this, use:

  • fflush(stdout) or cout.flush() in C++;
  • System.out.flush() in Java;
  • flush(output) in Pascal;
  • stdout.flush() in Python;
  • see documentation for other languages.

Examples: Two Houses Solution Codeforces


1 1 1


? 1 2
! 1 2


1 2 0 3


? 2 1
? 1 3
? 4 1
? 2 3
? 4 2
? 4 3
! 0 0


In the first sample input, we are given a city of three houses with one incoming road each. The user program asks one query: “? 1 2”: asking whether we can reach from house 11 to house 22. The judge responds with “Yes”. The user program now concludes that this is sufficient information to determine the correct answer. So, it outputs “! 1 2” and quits.

In the second sample input, the user program queries for six different pairs of houses, and finally answers “! 0 0” as it is convinced that no two houses as desired in the question exist in this city.


#include <bits/stdc++.h>

using namespace std; 

typedef long long int ll;

typedef unsigned long long int ull;

typedef unsigned long int ul;

typedef long double ld;

typedef double d;

typedef pair <int, int> ii;

typedef pair <ll, ll> pll;

typedef vector<ll> vll;

typedef vector<int> vi;

typedef priority_queue<ll> pqll;

typedef priority_queue<int> pqi;

typedef set<ll> sll;

typedef set<int> si;

typedef unordered_set<int> usi;

typedef unordered_set<ll> usll;

typedef multiset<ll> msll;

typedef multiset<int> msi;

typedef unordered_multiset<int> umsi;

typedef unordered_multiset<ll> umsll;

typedef map<ll, ll> mll;

typedef map<int, int> mi;

typedef unordered_map<int, int> umi;

typedef unordered_map<ll, ll> umll;

typedef multimap<ll, ll> mmll;

typedef multimap<int, int> mmi;

typedef unordered_multimap<int, int> ummi;

typedef unordered_multimap<ll, ll> ummll;

#define speed ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define pb push_back
#define pf push_front
#define mp make_pair
#define fs first
#define sd second
#define all(x) x.begin(), x.end()
#define REP(i, a, b) for (ll i=a; i=b; i–)
#define TRAV(a, v) for (auto a : v)
#define file ifstream cin (“Input.txt”); ofstream cout (“Output.txt”);

const long long int MOD = 1e9+7;

const long long int INF = 1e18;

const long double EPS = 1e-9;

int main()



int n;

cin >> n;

ii k[n];

REP(i, 0, n)


cin >> k[i].fs;

k[i].sd = i+1;


sort (k, k+n);

string resposta;

for (int i=n-1; i>=0; i–)


if (i == k[i].fs)


for (int j=0; j<i; j++)


cout << “? “ << k[i].sd << ‘ ‘ << k[j].sd << endl << flush;

cin >> resposta;

if (resposta[0] == ‘Y’)


cout << “! “ << k[i].sd << ‘ ‘ << k[j].sd << endl;

return 0;





cout << “! 0 0\n”;

return 0;


Also Read

Two Houses Solution Codeforces

4 thoughts on “[Solution] Two Houses Codeforces Round #711”

Leave a Comment