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First N natural can be divided into two sets with given difference and co-prime sums

Given N and M, task is to find whether numbers 1 to N can be divided into two sets such that the absolute difference between the sum of two sets is M and gcd of the sum of two sets is 1 (i.e. Sum of both sets are co-prime).

Prerequisite : GCD in CPP | GCD

Examples :

Input : N = 5 and M = 7
Output : YES
Explanation : as numbers from 1 to 5 can be divided into two sets {1, 2, 3, 5} and {4} such that absolute difference between the sum of both sets is 11 – 4 = 7 which is equal to M and also GCD(11, 4) = 1.

Input : N = 6 and M = 3
Output : NO
Explanation : In this case, Numbers from 1 to 6 can be divided into two sets {1, 2, 4, 5} and {3, 6} such that absolute difference between their sum is 12 – 9 = 3. But, since 12 and 9 are not co-prime as GCD(12, 9) = 3, the answer is ‘NO’.



Approach : Since we have 1 to N numbers, we know that the sum of all the numbers is N * (N + 1) / 2. Let S1 and S2 be two sets such that,
1) sum(S1) + sum(S2) = N * (N + 1) / 2
2) sum(S1) – sum(S2) = M
Solving these two equations will give us the sum of both the sets. If sum(S1) and sum(S2) are integers and they are co-prime (their GCD is 1), then there exists a way to split the numbers into two sets. Otherwise, there is no way to split these N numbers.

Below is the implementation of the solution described above.

C++

/* CPP code to determine whether numbers
   1 to N can be divided into two sets
   such that absolute difference between 
   sum of these two sets is M and these
   two sum are co-prime*/
#include <bits/stdc++.h>
using namespace std;
  
// function that returns boolean value
// on the basis of whether it is possible
// to divide 1 to N numbers into two sets
// that satisfy given conditions.
bool isSplittable(int n, int m)
{
    // initializing total sum of 1
    // to n numbers
    int total_sum = (n * (n + 1)) / 2;
  
    // since (1) total_sum = sum_s1 + sum_s2
    // and (2) m = sum_s1 - sum_s2
    // assuming sum_s1 > sum_s2.
    // solving these 2 equations to get
    // sum_s1 and sum_s2
    int sum_s1 = (total_sum + m) / 2;
  
    // total_sum = sum_s1 + sum_s2
    // and therefore
    int sum_s2 = total_sum - sum_s1;
  
    // if total sum is less than the absolute
    // difference then there is no way we
    // can split n numbers into two sets
    // so return false
    if (total_sum < m)
        return false;
  
    // check if these two sums are
    // integers and they add up to
    // total sum and also if their
    // absolute difference is m.
    if (sum_s1 + sum_s2 == total_sum &&
        sum_s1 - sum_s2 == m)
  
        // Now if two sum are co-prime
        // then return true, else return false.
        return (__gcd(sum_s1, sum_s2) == 1);
  
    // if two sums don't add up to total
    // sum or if their absolute difference
    // is not m, then there is no way to
    // split n numbers, hence return false
    return false;
}
  
// Driver code
int main()
{
    int n = 5, m = 7;
  
    // function call to determine answer
    if (isSplittable(n, m))
        cout << "Yes";
    else
        cout << "No";
  
    return 0;
}

Java

/* Java code to determine whether numbers
1 to N can be divided into two sets
such that absolute difference between 
sum of these two sets is M and these
two sum are co-prime*/
class GFG 
{
    static int GCD (int a, int b)
    {
        return b == 0 ? a : GCD(b, a % b);
    }
      
    // function that returns boolean value
    // on the basis of whether it is possible
    // to divide 1 to N numbers into two sets
    // that satisfy given conditions.
    static boolean isSplittable(int n, int m)
    {
          
        // initializing total sum of 1
        // to n numbers
        int total_sum = (n * (n + 1)) / 2;
      
        // since (1) total_sum = sum_s1 + sum_s2
        // and (2) m = sum_s1 - sum_s2
        // assuming sum_s1 > sum_s2.
        // solving these 2 equations to get
        // sum_s1 and sum_s2
        int sum_s1 = (total_sum + m) / 2;
      
        // total_sum = sum_s1 + sum_s2
        // and therefore
        int sum_s2 = total_sum - sum_s1;
      
        // if total sum is less than the absolute
        // difference then there is no way we
        // can split n numbers into two sets
        // so return false
        if (total_sum < m)
            return false;
      
        // check if these two sums are
        // integers and they add up to
        // total sum and also if their
        // absolute difference is m.
        if (sum_s1 + sum_s2 == total_sum &&
                    sum_s1 - sum_s2 == m)
      
            // Now if two sum are co-prime
            // then return true, else return false.
            return (GCD(sum_s1, sum_s2) == 1);
  
        // if two sums don't add up to total
        // sum or if their absolute difference
        // is not m, then there is no way to
        // split n numbers, hence return false
        return false;
    }
      
    // Driver Code
    public static void main(String args[]) 
    {
        int n = 5, m = 7;
  
        // function call to determine answer
        if (isSplittable(n, m))
            System.out.println("Yes");
        else
            System.out.println("No");
          
    }
}
  
// This code is contributed by Sam007

Python3

# Python3 code to determine whether numbers
# 1 to N can be divided into two sets
# such that absolute difference between
# sum of these two sets is M and these
# two sum are co-prime

def __gcd (a, b):

return a if(b == 0) else __gcd(b, a % b);

# function that returns boolean value
# on the basis of whether it is possible
# to divide 1 to N numbers into two sets
# that satisfy given conditions.
def isSplittable(n, m):

# initializing total sum of 1
# to n numbers
total_sum = (int)((n * (n + 1)) / 2);

# since (1) total_sum = sum_s1 + sum_s2
# and (2) m = sum_s1 – sum_s2
# assuming sum_s1 > sum_s2.
# solving these 2 equations to get
# sum_s1 and sum_s2
sum_s1 = int((total_sum + m) / 2);

# total_sum = sum_s1 + sum_s2
# and therefore
sum_s2 = total_sum – sum_s1;

# if total sum is less than the absolute
# difference then there is no way we
# can split n numbers into two sets
# so return false
if (total_sum < m): return False; # check if these two sums are # integers and they add up to # total sum and also if their # absolute difference is m. if (sum_s1 + sum_s2 == total_sum and sum_s1 - sum_s2 == m): # Now if two sum are co-prime # then return true, else return false. return (__gcd(sum_s1, sum_s2) == 1); # if two sums don't add up to total # sum or if their absolute difference # is not m, then there is no way to # split n numbers, hence return false return False; # Driver code n = 5; m = 7; # function call to determine answer if (isSplittable(n, m)): print("Yes"); else: print("No"); # This code is contributed by mits [tabby title="C#"]

/* C# code to determine whether numbers
1 to N can be divided into two sets
such that absolute difference between 
sum of these two sets is M and these
two sum are co-prime*/
using System;
  
class GFG {
  
    static int GCD (int a, int b)
    {
        return b == 0 ? a : GCD(b, a % b);
    }
      
    // function that returns boolean value
    // on the basis of whether it is possible
    // to divide 1 to N numbers into two sets
    // that satisfy given conditions.
    static bool isSplittable(int n, int m)
    {
          
        // initializing total sum of 1
        // to n numbers
        int total_sum = (n * (n + 1)) / 2;
      
        // since (1) total_sum = sum_s1 + sum_s2
        // and (2) m = sum_s1 - sum_s2
        // assuming sum_s1 > sum_s2.
        // solving these 2 equations to get
        // sum_s1 and sum_s2
        int sum_s1 = (total_sum + m) / 2;
      
        // total_sum = sum_s1 + sum_s2
        // and therefore
        int sum_s2 = total_sum - sum_s1;
      
        // if total sum is less than the absolute
        // difference then there is no way we
        // can split n numbers into two sets
        // so return false
        if (total_sum < m)
            return false;
      
        // check if these two sums are
        // integers and they add up to
        // total sum and also if their
        // absolute difference is m.
        if (sum_s1 + sum_s2 == total_sum &&
                       sum_s1 - sum_s2 == m)
      
            // Now if two sum are co-prime
            // then return true, else return false.
            return (GCD(sum_s1, sum_s2) == 1);
  
        // if two sums don't add up to total
        // sum or if their absolute difference
        // is not m, then there is no way to
        // split n numbers, hence return false
        return false;
    }
      
    // Driver code
    public static void Main()
    {
        int n = 5, m = 7;
  
        // function call to determine answer
        if (isSplittable(n, m))
            Console.Write("Yes");
        else
            Console.Write("No");
    }
}
  
// This code is contributed by Sam007.

PHP

<?php
/* PHP code to determine whether numbers
1 to N can be divided into two sets
such that absolute difference between 
sum of these two sets is M and these
two sum are co-prime*/
  
function __gcd ($a, $b)
{
        return $b == 0 ? $a : __gcd($b, $a % $b);
}
  
// function that returns boolean value
// on the basis of whether it is possible
// to divide 1 to N numbers into two sets
// that satisfy given conditions.
function isSplittable($n, $m)
{
    // initializing total sum of 1
    // to n numbers
    $total_sum = (int)(($n * ($n + 1)) / 2);
  
    // since (1) total_sum = sum_s1 + sum_s2
    // and (2) m = sum_s1 - sum_s2
    // assuming sum_s1 > sum_s2.
    // solving these 2 equations to get
    // sum_s1 and sum_s2
    $sum_s1 = (int)(($total_sum + $m) / 2);
  
    // total_sum = sum_s1 + sum_s2
    // and therefore
    $sum_s2 = $total_sum - $sum_s1;
  
    // if total sum is less than the absolute
    // difference then there is no way we
    // can split n numbers into two sets
    // so return false
    if ($total_sum < $m)
        return false;
  
    // check if these two sums are
    // integers and they add up to
    // total sum and also if their
    // absolute difference is m.
    if ($sum_s1 + $sum_s2 == $total_sum &&
        $sum_s1 - $sum_s2 == $m)
  
        // Now if two sum are co-prime
        // then return true, else return false.
        return (__gcd($sum_s1, $sum_s2) == 1);
  
    // if two sums don't add up to total
    // sum or if their absolute difference
    // is not m, then there is no way to
    // split n numbers, hence return false
    return false;
}
  
// Driver code
$n = 5;
$m = 7;
  
// function call to determine answer
if (isSplittable($n, $m))
    echo "Yes";
else
    echo "No";
  
// This Code is Contributed by mits
?>

Output:

Yes

Time Complexity : O(log(n))



This article is attributed to GeeksforGeeks.org

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