Write an Efficient Method to Check if a Number is Multiple of 3

Write an Efficient Method to Check if a Number is Multiple of 3

Last Updated : 29 Mar, 2025

Try it on GfG Practice

Given an integer n, the task is to determine whether n is a multiple of 3 or not.

Examples:

Input: n = 9
Output: Yes
Explanation: 9 is divisible by 3, so it is a multiple of 3.
Input: n = 14
Output: No
Explanation: 14 is not divisible by 3, so it is not a multiple of 3.
Input: n = 21
Output: Yes
Explanation: 21 is divisible by 3, so it is a multiple of 3.

Table of Content

[Approach 1] Using Repeated Sum of Digits - O(log(n)) Time and O(1) Space
[Approach 2] Using Binary Representation - O(log(n)) Time and O(log(n)) Space
[Approach 3] Using Modulo Division - O(1) Time and O(1) Space

[Approach 1] Using Repeated Sum of Digits - O(log(n)) Time and O(1) Space

The idea is based on the divisibility rule of 3, which states that a number is a multiple of 3 if the sum of its digits is also a multiple of 3. Instead of directly checking divisibility, we repeatedly sum the digits until we get a single-digit number. If the final result is 3, 6, or 9, the number is divisible by 3; otherwise, it is not.

C++

// C++ program to check if a number is a
// multiple of 3 using Repeated Sum of Digits
#include <bits/stdc++.h>
using namespace std;

// Function to compute sum of digits
int sumOfDigits(int n) {
    int sum = 0;
    
    while (n > 0) {
        
        // Add last digit to sum
        sum += n % 10;
        
        // Remove last digit
        n /= 10;
    }
    
    return sum;
}

// Function to check if n is a multiple of 3
bool isMultipleOf3(int n) {
    
    // Make n positive
    if (n < 0) {
        n = -n;
    }
    
    while (n > 9) {
        
        // Replace n with sum of its digits
        n = sumOfDigits(n);
    }
    
    // Check if final result is 3, 6, or 9
    return (n == 3 || n == 6 || n == 9);
}

int main() {
    
    int n = 9;
 
    if (isMultipleOf3(n)) {
        cout << "Yes" << endl;
    } 
    else {
        cout << "No" << endl;
    }
    
    return 0;
}

Java

// Java program to check if a number is a
// multiple of 3 using Repeated Sum of Digits
class GfG {

    // Function to compute sum of digits
    static int sumOfDigits(int n) {
        int sum = 0;

        while (n > 0) {

            // Add last digit to sum
            sum += n % 10;

            // Remove last digit
            n /= 10;
        }

        return sum;
    }

    // Function to check if n is a multiple of 3
    static boolean isMultipleOf3(int n) {

        // Make n positive
        if (n < 0) {
            n = -n;
        }

        while (n > 9) {

            // Replace n with sum of its digits
            n = sumOfDigits(n);
        }

        // Check if final result is 3, 6, or 9
        return (n == 3 || n == 6 || n == 9);
    }

    public static void main(String[] args) {

        int n = 9;

        if (isMultipleOf3(n)) {
            System.out.println("Yes");
        } 
        else {
            System.out.println("No");
        }
    }
}

Python

# Python program to check if a number is a
# multiple of 3 using Repeated Sum of Digits

# Function to compute sum of digits
def sumOfDigits(n):
    sum = 0

    while n > 0:

        # Add last digit to sum
        sum += n % 10

        # Remove last digit
        n //= 10

    return sum

# Function to check if n is a multiple of 3
def isMultipleOf3(n):

    # Make n positive
    if n < 0:
        n = -n

    while n > 9:

        # Replace n with sum of its digits
        n = sumOfDigits(n)

    # Check if final result is 3, 6, or 9
    return (n == 3 or n == 6 or n == 9)

if __name__ == "__main__":

    n = 9

    if isMultipleOf3(n):
        print("Yes")
    else:
        print("No")

C#

// C# program to check if a number is a
// multiple of 3 using Repeated Sum of Digits
using System;

class GfG {

    // Function to compute sum of digits
    static int sumOfDigits(int n) {
        int sum = 0;

        while (n > 0) {

            // Add last digit to sum
            sum += n % 10;

            // Remove last digit
            n /= 10;
        }

        return sum;
    }

    // Function to check if n is a multiple of 3
    static bool isMultipleOf3(int n) {

        // Make n positive
        if (n < 0) {
            n = -n;
        }

        while (n > 9) {

            // Replace n with sum of its digits
            n = sumOfDigits(n);
        }

        // Check if final result is 3, 6, or 9
        return (n == 3 || n == 6 || n == 9);
    }

    static void Main() {

        int n = 9;

        if (isMultipleOf3(n)) {
            Console.WriteLine("Yes");
        } 
        else {
            Console.WriteLine("No");
        }
    }
}

JavaScript

// JavaScript program to check if a number is a
// multiple of 3 using Repeated Sum of Digits

// Function to compute sum of digits
function sumOfDigits(n) {
    let sum = 0;

    while (n > 0) {

        // Add last digit to sum
        sum += n % 10;

        // Remove last digit
        n = Math.floor(n / 10);
    }

    return sum;
}

// Function to check if n is a multiple of 3
function isMultipleOf3(n) {

    // Make n positive
    if (n < 0) {
        n = -n;
    }

    while (n > 9) {

        // Replace n with sum of its digits
        n = sumOfDigits(n);
    }

    // Check if final result is 3, 6, or 9
    return (n == 3 || n == 6 || n == 9);
}

let n = 9;

if (isMultipleOf3(n)) {
    console.log("Yes");
} 
else {
    console.log("No");
}

Output

Yes

[Approach 2] Using Binary Representation - O(log(n)) Time and O(log(n)) Space

The idea is based on the binary representation of a number. The observation is that even powers of 2 contribute (3k+1) and odd powers contribute (3k-1) modulo 3. By counting set bits at odd and even positions and checking their difference, we determine if the number is divisible by 3. The approach uses recursion, reducing n by half in each step.

Mathematical Proof:

Consider a number n represented in binary form as abcde. Its decimal equivalent is:

n = (2^4 * a) + (2^3 * b) + (2^2 * c) + (2^1 * d) + (2^0 * e)

Observing powers of 2 modulo 3, we derive:

Even powers of 2 can be expressed as 3k + 1 (e.g., 2^0 = 1, 2^2 = 3k + 1).
Odd powers of 2 can be expressed as 3k - 1 (e.g., 2^1 = 3k - 1, 2^3 = 3k - 1).

Rewriting the decimal representation in terms of multiples of 3, we get:

n = (3k+1) * a + (3k-1) * b + (3k+1) * c + (3k-1) * d + (3k+1) * e

Rearranging the terms:

n = (3k)(a+b+c+d+e) + (a + c + e) - (b + d)

Since the first term is always a multiple of 3, for n to be a multiple of 3, the expression (a + c + e) - (b + d) must also be a multiple of 3. Thus, a number is divisible by 3 if and only if the difference between the count of set bits at odd positions and even positions is a multiple of 3.

C++

// C++ program to check if a number is a
// multiple of 3 using Binary 
// Representation
#include <bits/stdc++.h>
using namespace std;

// Function to check if n is a multiple of 3
bool isMultipleOf3(int n) {
    
    // Make n positive
    if (n < 0) {
        n = -n;
    }
    
    // Base cases
    if (n == 0) {
        return true;
    }
    if (n == 1) {
        return false;
    }
    
    int odd_count = 0, even_count = 0;
    
    while (n) {
        
        // If rightmost bit is set, increment odd_count
        if (n & 1) {
            odd_count++;
        }
        
        // Right shift n
        n >>= 1;
        
        // If rightmost bit is set, increment even_count
        if (n & 1) {
            even_count++;
        }
        
        // Right shift n again
        n >>= 1;
    }
    
    // Recursive call with the difference
    return isMultipleOf3(abs(odd_count - even_count));
}

int main() {
    
    int n = 9;
 
    if (isMultipleOf3(n)) {
        cout << "Yes" << endl;
    } 
    else {
        cout << "No" << endl;
    }
    
    return 0;
}

Java

// Java program to check if a number is a
// multiple of 3 using Binary 
// Representation
class GfG {

    // Function to check if n is a multiple of 3
    static boolean isMultipleOf3(int n) {
        
        // Make n positive
        if (n < 0) {
            n = -n;
        }
        
        // Base cases
        if (n == 0) {
            return true;
        }
        if (n == 1) {
            return false;
        }
        
        int odd_count = 0, even_count = 0;
        
        while (n > 0) {
            
            // If rightmost bit is set, increment odd_count
            if ((n & 1) == 1) {
                odd_count++;
            }
            
            // Right shift n
            n >>= 1;
            
            // If rightmost bit is set, increment even_count
            if ((n & 1) == 1) {
                even_count++;
            }
            
            // Right shift n again
            n >>= 1;
        }
        
        // Recursive call with the difference
        return isMultipleOf3(Math.abs(odd_count - even_count));
    }

    public static void main(String[] args) {
        
        int n = 9;
     
        if (isMultipleOf3(n)) {
            System.out.println("Yes");
        } 
        else {
            System.out.println("No");
        }
    }
}

Python

# Python program to check if a number is a
# multiple of 3 using Binary 
# Representation

# Function to check if n is a multiple of 3
def isMultipleOf3(n):
    
    # Make n positive
    if n < 0:
        n = -n
    
    # Base cases
    if n == 0:
        return True
    if n == 1:
        return False
    
    odd_count = 0
    even_count = 0
    
    while n:
        
        # If rightmost bit is set, increment odd_count
        if n & 1:
            odd_count += 1
        
        # Right shift n
        n >>= 1
        
        # If rightmost bit is set, increment even_count
        if n & 1:
            even_count += 1
        
        # Right shift n again
        n >>= 1
    
    # Recursive call with the difference
    return isMultipleOf3(abs(odd_count - even_count))

if __name__ == "__main__":
    
    n = 9
 
    if isMultipleOf3(n):
        print("Yes")
    else:
        print("No")

C#

// C# program to check if a number is a
// multiple of 3 using Binary 
// Representation
using System;

class GfG {
    
    // Function to check if n is a multiple of 3
    static bool isMultipleOf3(int n) {
        
        // Make n positive
        if (n < 0) {
            n = -n;
        }
        
        // Base cases
        if (n == 0) {
            return true;
        }
        if (n == 1) {
            return false;
        }
        
        int odd_count = 0, even_count = 0;
        
        while (n > 0) {
            
            // If rightmost bit is set, increment odd_count
            if ((n & 1) == 1) {
                odd_count++;
            }
            
            // Right shift n
            n >>= 1;
            
            // If rightmost bit is set, increment even_count
            if ((n & 1) == 1) {
                even_count++;
            }
            
            // Right shift n again
            n >>= 1;
        }
        
        // Recursive call with the difference
        return isMultipleOf3(Math.Abs(odd_count - even_count));
    }

    public static void Main() {
        
        int n = 9;
     
        if (isMultipleOf3(n)) {
            Console.WriteLine("Yes");
        } 
        else {
            Console.WriteLine("No");
        }
    }
}

JavaScript

// JavaScript program to check if a number is a
// multiple of 3 using Binary 
// Representation

// Function to check if n is a multiple of 3
function isMultipleOf3(n) {
    
    // Make n positive
    if (n < 0) {
        n = -n;
    }
    
    // Base cases
    if (n === 0) {
        return true;
    }
    if (n === 1) {
        return false;
    }
    
    let odd_count = 0, even_count = 0;
    
    while (n > 0) {
        
        // If rightmost bit is set, increment odd_count
        if (n & 1) {
            odd_count++;
        }
        
        // Right shift n
        n >>= 1;
        
        // If rightmost bit is set, increment even_count
        if (n & 1) {
            even_count++;
        }
        
        // Right shift n again
        n >>= 1;
    }
    
    // Recursive call with the difference
    return isMultipleOf3(Math.abs(odd_count - even_count));
}

let n = 9;

if (isMultipleOf3(n)) {
    console.log("Yes");
} 
else {
    console.log("No");
}

Output

Yes

[Approach 3] Using Modulo Division - O(1) Time and O(1) Space

The idea is to use the modulo operation to check divisibility by 3 efficiently. A number n is a multiple of 3 if n % 3 == 0, meaning it leaves no remainder when divided by 3.

C++

// C++ program to check if a number is a
// multiple of 3 using Modulo Division
#include <bits/stdc++.h>
using namespace std;

// Function to check if n is a multiple of 3
bool isMultipleOf3(int n) {
    return (n % 3 == 0);
}

int main() {
    
    int n = 9;
 
    if (isMultipleOf3(n)) {
        cout << "Yes" << endl;
    } 
    else {
        cout << "No" << endl;
    }
    
    return 0;
}

Java

// Java program to check if a number is a
// multiple of 3 using Modulo Division
class GfG {
    
    // Function to check if n is a multiple of 3
    static boolean isMultipleOf3(int n) {
        return (n % 3 == 0);
    }

    public static void main(String[] args) {
        
        int n = 9;
    
        if (isMultipleOf3(n)) {
            System.out.println("Yes");
        } 
        else {
            System.out.println("No");
        }
    }
}

Python

# Python program to check if a number is a
# multiple of 3 using Modulo Division

# Function to check if n is a multiple of 3
def isMultipleOf3(n):
    return (n % 3 == 0)

if __name__ == "__main__":
    
    n = 9

    if isMultipleOf3(n):
        print("Yes")
    else:
        print("No")

C#

// C# program to check if a number is a
// multiple of 3 using Modulo Division
using System;

class GfG {
    
    // Function to check if n is a multiple of 3
    static bool isMultipleOf3(int n) {
        return (n % 3 == 0);
    }

    static void Main() {
        
        int n = 9;
    
        if (isMultipleOf3(n)) {
            Console.WriteLine("Yes");
        } 
        else {
            Console.WriteLine("No");
        }
    }
}

JavaScript

// JavaScript program to check if a number is a
// multiple of 3 using Modulo Division

// Function to check if n is a multiple of 3
function isMultipleOf3(n) {
    return (n % 3 === 0);
}

// Driver Code
let n = 9;

if (isMultipleOf3(n)) {
    console.log("Yes");
} 
else {
    console.log("No");
}

Output

Yes

Write an Efficient Method to Check if a Number is Multiple of 3

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