📺 Info Siaran: competitive programming/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.

Approach 1

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

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

JavaScript

// Setting the lower and upper bounds
let lowerBound = 2;
let upperBound = 10;

// Number to be guessed is 6

// Iterating from lowerBound to upperBound
for (let i = lowerBound; i <= upperBound; i++) {
    console.log(i);

    // Simulating input from the judge
    // (For actual input, you can use prompt() in a browser environment)
    let response = prompt("Enter 0 if the number is guessed correctly:");

    // Converting the response to an integer
    response = parseInt(response);

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

// This code is contributed by divyeshrabadiya07

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