Check if Array Elements can be Made Equal with Given Operations

Given an array arr[] consisting of N integers and following are the three operations that can be performed using any external number X.

1. Add X to an array element once.
2. Subtract X from an array element once.
3. Perform no operation on the array element.

Note : Only one operation can be performed on a number and only once.

The task is to check whether there exists a number X, such that if the above operations are performed with the number X, the array elements become equal. If the number exists, print "Yes". Otherwise, print "No".

Examples:

Input: arr[] = {2, 3, 3, 4, 2}
Output: Yes
Explanation:
Consider the value of X as 1 and increment the array element arr[0](= 2) and arr[4](= 2) by 1, and decrement arr[3](= 4) by 1 modifies the array to {3, 3, 3, 3, 3}.
Therefore, print Yes with the value X as 1.

Input: arr[] = {4, 3, 2, 1}
Output: No

The given problem can be solved based on the following observations:

• If all numbers are equal, the answer is "YES".
• If there are two distinct numbers in the array, the answer is "YES", as every distinct number can be converted to another integer by either adding the difference to smaller numbers or subtracting the difference from the larger numbers.
• If there are three distinct numbers A < B < C, then all array elements can be made equal by incrementing all the As by B - A and decrementing all the Cs by C - A only when (B - A) = (C - B) or B equals (C + A) / 2.
• If there are more than 3 distinct numbers in the array, the answer is "NO", because of the property of the addition.

The idea is to use a hash set to store all unique elements of the array, then check the above conditions on the unique elements stored in the hash.

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

// Returns true if we can equalize arr using some x
bool canEqualise(vector<int>& arr)
{
    // Collect all unique elements using unordered_set
    unordered_set<int> s(arr.begin(), arr.end());
    
    // If there is only one or two unique
    // values
    if (s.size() == 1 || s.size() == 2)
        return true;

    // If three unique elements, check if they
    // form an arithmetic sequence
    else if (s.size() == 3) {
      
        // Sort to get min, mid, max
        vector<int> unq(s.begin(), s.end());
        sort(unq.begin(), unq.end());
        
        int mn = unq[0];
        int md = unq[1];
        int mx = unq[2];
        
        // Check if the differences are equal
        // to form an arithmetic sequence
        if ((mx - md) == (md - mn)) {
            return true;
        }
    }

    // More than three unique elements: cannot equalize
    return false;
}
  
// Driver code
int main()
{
    vector<int> arr = {55, 52, 52, 49, 52};
    if (canEqualise(arr))
        cout << "YES" << endl;
    else
        cout << "NO" << endl;
}

import java.util.*;

// Returns true if we can equalize arr using some x
public class Main {
    public static boolean canEqualise(int[] arr) {
      
        // Collect all unique elements using HashSet
        Set<Integer> s = new HashSet<>();
        for (int num : arr) {
            s.add(num);
        }

        // If there is only one or two unique values
        if (s.size() == 1 || s.size() == 2)
            return true;

        // If three unique elements, check if they
        // form an arithmetic sequence
        else if (s.size() == 3) {
          
            // Sort to get min, mid, max
            List<Integer> unq = new ArrayList<>(s);
            Collections.sort(unq);
            int mn = unq.get(0);
            int md = unq.get(1);
            int mx = unq.get(2);

            // Check if the differences are equal 
            // to form an arithmetic sequence
            if ((mx - md) == (md - mn)) {
                return true;
            }
        }

        // More than three unique elements: cannot equalize
        return false;
    }

    // Driver code
    public static void main(String[] args) {
        int[] arr = {55, 52, 52, 49, 52};
        if (canEqualise(arr))
            System.out.println("YES");
        else
            System.out.println("NO");
    }
}

def canEqualise(arr):
    # Collect all unique elements using set
    s = set(arr)

    # If there is only one or two unique values
    if len(s) == 1 or len(s) == 2:
        return True

    # If three unique elements, check if they 
    # form an arithmetic sequence
    elif len(s) == 3:
      
        # Sort to get min, mid, max
        unq = sorted(s)
        mn = unq[0]
        md = unq[1]
        mx = unq[2]

        # Check if the differences are equal 
        # to form an arithmetic sequence
        if (mx - md) == (md - mn):
            return True

    # More than three unique elements: 
    # cannot equalize
    return False

# Driver code
arr = [55, 52, 52, 49, 52]
if canEqualise(arr):
    print("YES")
else:
    print("NO")

// Returns true if we can equalize arr using some x
using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    public static bool CanEqualise(List<int> arr)
    {
        // Collect all unique elements using HashSet
        HashSet<int> s = new HashSet<int>(arr);
        
        // If there is only one or two unique values
        if (s.Count == 1 || s.Count == 2)
            return true;
        
        // If three unique elements, check if they form an arithmetic sequence
        else if (s.Count == 3)
        {
            // Sort to get min, mid, max
            int[] unq = s.ToArray();
            Array.Sort(unq);
            
            int mn = unq[0];
            int md = unq[1];
            int mx = unq[2];
            
            // Check if the differences are equal to form an arithmetic sequence
            if ((mx - md) == (md - mn))
            {
                return true;
            }
        }
        
        // More than three unique elements: cannot equalize
        return false;
    }
    
    // Driver code
    static void Main()
    {
        List<int> arr = new List<int> { 55, 52, 52, 49, 52 };
        if (CanEqualise(arr))
            Console.WriteLine("YES");
        else
            Console.WriteLine("NO");
    }
}

// Returns true if we can equalize arr using some x
function canEqualise(arr) {
    // Collect all unique elements using Set
    const s = new Set(arr);
    
    // If there is only one or two unique values
    if (s.size === 1 || s.size === 2) return true;
    
    // If three unique elements, check if they form an arithmetic sequence
    else if (s.size === 3) {
        // Sort to get min, mid, max
        const unq = Array.from(s).sort((a, b) => a - b);
        
        const mn = unq[0];
        const md = unq[1];
        const mx = unq[2];
        
        // Check if the differences are equal to form an arithmetic sequence
        if ((mx - md) === (md - mn)) {
            return true;
        }
    }
    
    // More than three unique elements: cannot equalize
    return false;
}
  
// Driver code
const arr = [55, 52, 52, 49, 52];
if (canEqualise(arr))
    console.log('YES');
else
    console.log('NO');

YES

Time Complexity : O(n)
Auxiliary Space : O(n)

Further Optimization : Since we care about only 3 distinct elements, we can avoid the use of hash table and use three variables. Hence we can solve this problem in O(n) Time and O(1) Space. Please refer Check it is possible to make array elements same using one external number for details