Interactive Problems in Competitive Programming

Interactive Problems are those problems in which our solution or code interacts with the judge in real time. When we develop a solution for an Interactive Problem then the input data given to our solution may not be predetermined but is built for that problem specifically. The solution performs a series of exchange of data with the judge and at the end of the conversation the judge decides whether our solution was correct or not.

Guessing the Number (An Interactive Problem)

In this problem the user has to guess the number during a communication with the judge. The user is provided with the upper and lower bound and he/she can ask the judge whether a number is the number to be guessed. The judge replies with -1 if the number is smaller than the number to be guessed or 1 if number is greater than the number to be guessed or 0 if it is equal to the number to be guessed.

Approach 1 : Linear Guessing 

The user can query the judge for all the numbers between lower limit and upper limit to find the solution. 

C++

#include <bits/stdc++.h>
using namespace std;

int main()
{
    int lower_bound = 2;
    int upper_bound = 10;

    // Number to be guessed is 6

    // Iterating from lower_bound to upper_bound
    for (int i = lower_bound; i <= upper_bound; i++) {
        cout << i << endl;

        // Input the response from the judge
        int response;
        cin >> response;

        if (response == 0) {
            cout << "Number guessed is :" << i;
            break;
        }
    }

    return 0;
}

// This code is contributed by divyeshrabadiya07

Java

import java.util.*;
class GFG {
    public static void main(String[] args)
    {
        Scanner sc1 = new Scanner(System.in);
        int lower_bound = 2;
        int upper_bound = 10;

        // Number to be guessed is 6

        // Iterating from lower_bound to upper_bound
        for (int i = lower_bound; i <= upper_bound; i++) {
            System.out.println(i);

            // Input the response from the judge
            int response = sc1.nextInt();

            if (response == 0) {
                System.out.println("Number guessed is :" + i);
                break;
            }
        }
    }
}

Python3

if __name__=='__main__':
    lower_bound = 2;
    upper_bound = 10;

    # Number to be guessed is 6

    # Iterating from lower_bound to upper_bound
    for i in range(lower_bound, upper_bound + 1):
        print(i)

        # Input the response from the judge
        response = int(input())

        if (response == 0):
            print("Number guessed is :", i, end = '')
            break;

            # This code is contributed by rutvik_56

C#

using System;
class GFG
{
    public static void Main(string[] args)
    {
        int lower_bound = 2;
        int upper_bound = 10;

        // Number to be guessed is 6

        // Iterating from lower_bound to upper_bound
        for (int i = lower_bound; i <= upper_bound; i++)
        {
            Console.WriteLine(i);

            // Input the response from the judge
            int response = int.Parse(Console.ReadLine());

            if (response == 0) {
                Console.WriteLine("Number guessed is :" + i);
                break;
            }
        }
    }
}

// This code is contributed by Pratham76

Output

2
Number guessed is :2

Time Complexity: O(n)

Approach 2 : Applying Binary Search 

We can also apply binary search interactively to find the solution. This solution is efficient as compared to the previous approach.

C++

#include <bits/stdc++.h>

using namespace std;

int main(int argc, char** argv)
{
    int lower_bound = 2;
    int upper_bound = 10;

    while (lower_bound <= upper_bound)
    {
        int mid = (lower_bound + upper_bound) / 2;
        cout << mid << endl;
        int response;
        cin >> response;

        if (response == -1)
            lower_bound = mid + 1;

        else if (response == 1)
            upper_bound = mid - 1;

        else if (response == 0){
            cout << "Number guessed is :" + to_string(mid) << endl;
            break;
        }
    }
    return 0;
}

// This code is contributed by geeky01adarsh

Java

import java.util.*;
class GFG {
    public static void main(String[] args)
    {
        Scanner sc1 = new Scanner(System.in);
        int lower_bound = 2;
        int upper_bound = 10;

        // Number to be guessed is 9

        // Applying Binary Search interactively
        while (lower_bound <= upper_bound) {
            int mid = (lower_bound + upper_bound) / 2;

            // Print the guessed number
            System.out.println(mid);

            // Input the response from the judge
            int response = sc1.nextInt();

            if (response == -1) {
                lower_bound = mid + 1;
            }
            else if (response == 1) {
                upper_bound = mid - 1;
            }
            else if (response == 0) {
                System.out.println("Number guessed is :" + mid);
                break;
            }
        }
    }
}

C#

using System;
class GFG {
  static void Main() {
    int lower_bound = 2;
    int upper_bound = 10;

    // Number to be guessed is 9

    // Applying Binary Search interactively
    while (lower_bound <= upper_bound) {
        int mid = (lower_bound + upper_bound) / 2;

        // Print the guessed number
        Console.WriteLine(mid);

        // Input the response from the judge
        int response = Convert.ToInt32(Console.ReadLine());

        if (response == -1) {
            lower_bound = mid + 1;
        }
        else if (response == 1) {
            upper_bound = mid - 1;
        }
        else if (response == 0) {
            Console.WriteLine("Number guessed is :" + mid);
            break;
        }
    }
  }
}

// This code is contributed by divyesh072019

Python3

lower_bound = 2
upper_bound = 10

# Number to be guessed is 9

# Applying Binary Search interactively
while (lower_bound <= upper_bound) :
    mid = (lower_bound + upper_bound) // 2

    # Print guessed number
    print(mid)

    # Input the response from the judge
    response = int(input())

    if (response == -1) :
        lower_bound = mid + 1

    elif (response == 1) :
        upper_bound = mid - 1

    elif (response == 0) :
        print("Number guessed is :", mid)
        break

Output

6
Number guessed is :6

Time Complexity: O(logn) 
Algorithm Paradigm: Divide and Conquer