Length of the smallest number which is divisible by K and formed by using 1’s only

Given an integer K, the task is to find the length of the smallest no. N which is divisible by K and formed by using 1 as its digits only. If no such number exists then print -1
Examples:

Input: K = 3 
Output: 3 
111 is the smallest number formed by using 1 only 
which is divisible by 3.
Input: K = 7 
Output: 6 
111111 is the required number.
Input: K = 12 
Output: -1 

Naive approach:

1. First we have to check if K is a multiple of either 2 or 5 then the answer will be -1 because there is no number formed by using only 1’s as its digits which is divisible by 2 or 5.
2. Now iterate for every possible no. formed by using 1’s at most K times and check for its divisibility with K.

Below is the implementation of the above approach: 

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

// Function to return length
// of the resultant number
int numLen(int K)
{

    // If K is a multiple of 2 or 5
    if (K % 2 == 0 || K % 5 == 0)
        return -1;

    int number = 0;

    int len = 1;

    for (len = 1; len <= K; len++) {

        // Generate all possible numbers
        // 1, 11, 111, 111, ..., K 1's
        number = number * 10 + 1;

        // If number is divisible by k
        // then return the length
        if ((number % K == 0))
            return len;
    }

    return -1;
}

// Driver code
int main()
{

    int K = 7;
    cout << numLen(K);

    return 0;
}

Java

// Java implementation of the approach
class GFG
{

    // Function to return length
    // of the resultant number
    static int numLen(int K)
    {

        // If K is a multiple of 2 or 5
        if (K % 2 == 0 || K % 5 == 0)
        {
            return -1;
        }

        int number = 0;

        int len = 1;

        for (len = 1; len <= K; len++)
        {

            // Generate all possible numbers
            // 1, 11, 111, 111, ..., K 1's
            number = number * 10 + 1;

            // If number is divisible by k
            // then return the length
            if ((number % K == 0))
            {
                return len;
            }
        }

        return -1;
    }

    // Driver code
    public static void main(String[] args)
    {
        int K = 7;
        System.out.println(numLen(K));
    }
}

/* This code contributed by PrinciRaj1992 */

Python3

# Python implementation of the approach

# Function to return length
# of the resultant number
def numLen(K):

    # If K is a multiple of 2 or 5
    if (K % 2 == 0 or K % 5 == 0):
        return -1;

    number = 0;

    len = 1;

    for len in range(1,K+1):

        # Generate all possible numbers
        # 1, 11, 111, 111, ..., K 1's
        number = number * 10 + 1;

        # If number is divisible by k
        # then return the length
        if ((number % K == 0)):
            return len;

    return -1;

# Driver code
K = 7;
print(numLen(K));

# This code contributed by Rajput-Ji

C#

// C# implementation of the approach
using System;

class GFG
{

// Function to return length
// of the resultant number
static int numLen(int K)
{

    // If K is a multiple of 2 or 5
    if (K % 2 == 0 || K % 5 == 0)
        return -1;

    int number = 0;

    int len = 1;

    for (len = 1; len <= K; len++)
    {

        // Generate all possible numbers
        // 1, 11, 111, 111, ..., K 1's
        number = number * 10 + 1;

        // If number is divisible by k
        // then return the length
        if ((number % K == 0))
            return len;
    }

    return -1;
}

// Driver code
static void Main()
{
    int K = 7;
    Console.WriteLine(numLen(K));
}
}

// This code is contributed by mits

PHP

<?php
// PHP implementation of the approach

// Function to return length
// of the resultant number
function numLen($K)
{

    // If K is a multiple of 2 or 5
    if ($K % 2 == 0 || $K % 5 == 0)
        return -1;

    $number = 0;

    $len = 1;

    for ($len = 1; $len <= $K; $len++)
    {

        // Generate all possible numbers
        // 1, 11, 111, 111, ..., K 1's
        $number = $number * 10 + 1;

        // If number is divisible by k
        // then return the length
        if (($number % $K == 0))
            return $len;
    }

    return -1;
}

// Driver code
$K = 7;
echo numLen($K);

// This code is contributed by Akanksha Rai
?>

Javascript

<script>
// javascript implementation of the approach

// Function to return length
    // of the resultant number
    function numLen(K) {

        // If K is a multiple of 2 or 5
        if (K % 2 == 0 || K % 5 == 0) {
            return -1;
        }

        var number = 0;
        var len = 1;
        for (len = 1; len <= K; len++)
        {

            // Generate all possible numbers
            // 1, 11, 111, 111, ..., K 1's
            number = number * 10 + 1;

            // If number is divisible by k
            // then return the length
            if ((number % K == 0)) {
                return len;
            }
        }
        return -1;
    }

    // Driver code
    var K = 7;
    document.write(numLen(K));

// This code is contributed by Princi Singh
</script>

Output:

6

Time Complexity: O(K)

Auxiliary Space: O(1)

Efficient Approach: As we see in the above approach we generate all possible numbers like 1, 11, 1111, 11111, …, K times but if the value of K is very large then the no. will be out of range of data type so we can make use of the modular properties. 
Instead of doing number = number * 10 + 1, we can do better as number = (number * 10 + 1) % K
Explanation: We start with number = 1 and repeatedly do number = number * 10 + 1 then in each iteration we’ll get a new term of the above sequence. 

1*10 + 1 = 11 
11*10 + 1 = 111 
111*10 + 1 = 1111 
1111*10 + 1 = 11111 
11111*10 + 1 = 111111 

Since we are repeatedly taking the remainders of the number at each step, at each step we have, newNum = oldNum * 10 + 1 .By the rules of modular arithmetic (a * b + c) % m is same as ((a * b) % m + c % m) % m. So, it doesn’t matter whether oldNum is the remainder or the original number, the answer would be correct.
Below is the implementation of the above approach:

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

// Function to return length
// of the resultant number
int numLen(int K)
{

    // If K is a multiple of 2 or 5
    if (K % 2 == 0 || K % 5 == 0)
        return -1;

    int number = 0;

    int len = 1;

    for (len = 1; len <= K; len++) {

        // Instead of generating all possible numbers
        // 1, 11, 111, 111, ..., K 1's
        // Take remainder with K
        number = (number * 10 + 1) % K;

        // If number is divisible by k
        // then remainder will be 0
        if (number == 0)
            return len;
    }

    return -1;
}

// Driver code
int main()
{

    int K = 7;
    cout << numLen(K);

    return 0;
}

Java

// Java implementation of the approach
class GFG {

    // Function to return length
    // of the resultant number
    public static int numLen(int K)
    {

        // If K is a multiple of 2 or 5
        if (K % 2 == 0 || K % 5 == 0)
            return -1;

        int number = 0;

        int len = 1;

        for (len = 1; len <= K; len++) {

            // Instead of generating all possible numbers
            // 1, 11, 111, 111, ..., K 1's
            // Take remainder with K
            number = (number * 10 + 1) % K;

            // If number is divisible by k
            // then remainder will be 0
            if (number == 0)
                return len;
        }

        return -1;
    }

    // Driver code
    public static void main(String[] args)
    {
        int K = 7;
        System.out.print(numLen(K));
    }
}

Python3

# Python3 implementation of the approach

# Function to return length
# of the resultant number
def numLen(K):

    # If K is a multiple of 2 or 5
    if(K % 2 == 0 or K % 5 == 0):
        return -1

    number = 0

    len = 1

    for len in range (1, K + 1):

        # Instead of generating all possible numbers
        # 1, 11, 111, 111, ..., K 1's
        # Take remainder with K
        number = ( number * 10 + 1 ) % K

        # If number is divisible by k
        # then remainder will be 0
        if number == 0:
            return len

    return -1

# Driver code
K = 7
print(numLen(K))

C#

// C# implementation of the approach
using System;

class GFG
{

// Function to return length
// of the resultant number
public static int numLen(int K)
{

    // If K is a multiple of 2 or 5
    if (K % 2 == 0 || K % 5 == 0)
        return -1;

    int number = 0;

    int len = 1;

    for (len = 1; len <= K; len++)
    {

        // Instead of generating all possible numbers
        // 1, 11, 111, 111, ..., K 1's
        // Take remainder with K
        number = (number * 10 + 1) % K;

        // If number is divisible by k
        // then remainder will be 0
        if (number == 0)
            return len;
    }

    return -1;
}

// Driver code
public static void Main()
{
    int K = 7;
    Console.WriteLine(numLen(K));
}
}

// This code is contributed by Ryuga

PHP

<?php
// PHP implementation of the approach

// Function to return length
// of the resultant number
function numLen($K)
{

    // If K is a multiple of 2 or 5
    if ($K % 2 == 0 || $K % 5 == 0)
        return -1;

    $number = 0;

    $len = 1;

    for ($len = 1; $len <= $K; $len++)
    {

        // Instead of generating all possible numbers
        // 1, 11, 111, 111, ..., K 1's
        // Take remainder with K
        $number = ($number * 10 + 1) % $K;

        // If number is divisible by k
        // then remainder will be 0
        if ($number == 0)
            return $len;
    }

    return -1;
}

// Driver code
$K = 7;
echo numLen($K);

// This code is contributed by mits
?>

Javascript

<script>
// javascript implementation of the approach
// Function to return length
// of the resultant number
function numLen(K)
{

    // If K is a multiple of 2 or 5
    if (K % 2 == 0 || K % 5 == 0)
        return -1;

    var number = 0;

    var len = 1;

    for (len = 1; len <= K; len++) {

        // Instead of generating all possible numbers
        // 1, 11, 111, 111, ..., K 1's
        // Take remainder with K
        number = (number * 10 + 1) % K;

        // If number is divisible by k
        // then remainder will be 0
        if (number == 0)
            return len;
    }

    return -1;
}

// Driver code
var K = 7;
document.write(numLen(K));


// This code contributed by shikhasingrajput

</script>

Output:

6

Time Complexity: O(K) 
Auxiliary Space: O(1)