Skip to content
Related Articles

Related Articles

N-th multiple in sorted list of multiples of two numbers
  • Difficulty Level : Medium
  • Last Updated : 05 May, 2021

Given three positive integers a, b and n. Consider a list that has all multiples of ‘a’ and ‘b’. the list is sorted sorted and duplicates are removed. The task is to find n-th element of the list.
Examples : 
 

Input :  a = 3, b = 5, n = 5
Output : 10
a = 3 b = 5, Multiple of 3 are 3, 6, 9, 12, 15,... and 
multiples of 5 are 5, 10, 15, 20,.... 
After deleting duplicate element and sorting:
3, 5, 6, 9, 10, 12, 15, 18, 20,.... 
The 5th element in the sequence is 10.

Input : n = 6, a = 2, b = 3 
Output : 9

 

Method 1: (Brute Force) 
Generate the first ‘n’ multiple of ‘a’. Now, generate first ‘n’ multiple of b such that it do not belong to the first n multiple of ‘a’. This can be done using Binary Search. 
 

C++




// C++ program to find n-th number in the sorted
// list of multiples of two numbers.
#include<bits/stdc++.h>
using namespace std;
 
// Return the n-th number in the sorted
// list of multiples of two numbers.
int nthElement(int a, int b, int n)
{
    vector<int> seq;
 
    // Generating first n multiple of a.
    for (int i = 1; i <= n; i++)
        seq.push_back(a*i);
 
    // Sorting the sequence.
    sort(seq.begin(), seq.end());
 
    // Generating and storing first n multiple of b
    // and storing if not present in the sequence.
    for (int i = 1, k = n; i <= n && k; i++)
    {
        // If not present in the sequence
        if (!binary_search(seq.begin(), seq.end(), b*i))
        {
            // Storing in the sequence.
            seq.push_back(b*i);
 
            sort(seq.begin(), seq.end());
            k--;
        }
    }
 
    return seq[n - 1];
}
 
// Driven Program
int main()
{
    int a = 3, b = 5, n = 5;
    cout << nthElement(a, b, n) << endl;
    return 0;
}

Java




// Java program to find n-th number
// in the sorted list of multiples
// of two numbers.
import java.io.*;
import java.util.*;
class GFG
{
// Return the n-th number in the sorted
// list of multiples of two numbers.
static int nthElement(int a, int b, int n)
{
    ArrayList<Integer> seq = new
              ArrayList<Integer>(n * n + 1);
     
    // Generating first n multiple of a.
    for (int i = 1; i <= n; i++)
        seq.add(a * i);
         
    // Sorting the sequence.
    Collections.sort(seq);
     
    // Generating and storing first n
    // multiple of b and storing if
    // not present in the sequence.
    for (int i = 1, k = n;
             i <= n && k > 0; i++)
    {
        // If not present in the sequence
        if (seq.indexOf(b * i) == -1)
        {
            // Storing in the sequence.
            seq.add(b * i);
            Collections.sort(seq);
            k--;
        }
    }
    return seq.get(n - 1);
}
 
// Driver Code
public static void main(String[] args)
{
    int a = 3, b = 5, n = 5;
    System.out.println(nthElement(a, b, n));
}
}
 
// This code is contributed by mits

Python3




# Python3 program to find n-th
# number in the sorted list of
# multiples of two numbers.
 
# Return the n-th number in the
# sorted list of multiples of
# two numbers.
def nthElement(a, b, n):
    seq = [];
 
    # Generating first n
    # multiple of a.
    for i in range(1, n + 1):
        seq.append(a * i);
 
    # Sorting the sequence.
    seq.sort();
 
    # Generating and storing first
    # n multiple of b and storing
    # if not present in the sequence.
    i = 1;
    k = n;
    while(i <= n and k > 0):
         
        # If not present in the sequence
        try:
            z = seq.index(b * i);
        except ValueError:
             
            # Storing in the sequence.
            seq.append(b * i);
            seq.sort();
            k -= 1;
        i += 1;
 
    return seq[n - 1];
 
# Driver Code
a = 3;
b = 5;
n = 5;
print(nthElement(a, b, n));
 
# This code is contributed by mits

C#




// C# program to find n-th number
// in the sorted list of multiples
// of two numbers.
using System;
using System.Collections;
 
class GFG
{
// Return the n-th number in the sorted
// list of multiples of two numbers.
static int nthElement(int a,
                      int b, int n)
{
    ArrayList seq = new ArrayList();
     
    // Generating first n multiple of a.
    for (int i = 1; i <= n; i++)
        seq.Add(a * i);
 
    // Sorting the sequence.
    seq.Sort();
 
    // Generating and storing first n
    // multiple of b and storing if
    // not present in the sequence.
    for (int i = 1, k = n;
             i <= n && k > 0; i++)
    {
        // If not present in the sequence
        if (!seq.Contains(b * i))
        {
            // Storing in the sequence.
            seq.Add(b * i);
            seq.Sort();
            k--;
        }
    }
 
    return (int)seq[n - 1];
}
 
// Driver Code
static void Main()
{
    int a = 3, b = 5, n = 5;
    Console.WriteLine(nthElement(a, b, n));
}
}
 
// This code is contributed by mits

PHP




<?php
// PHP program to find n-th number
// in the sorted list of multiples
// of two numbers.
 
// Return the n-th number in the sorted
// list of multiples of two numbers.
function nthElement($a, $b, $n)
{
    $seq = array();
 
    // Generating first n multiple of a.
    for ($i = 1; $i <= $n; $i++)
        array_push($seq, $a * $i);
 
    // Sorting the sequence.
    sort($seq);
 
    // Generating and storing first
    // n multiple of b and storing
    // if not present in the sequence.
    for ($i = 1, $k = $n;
         $i <= $n && $k > 0; $i++)
    {
        // If not present in the sequence
        if (array_search($b * $i, $seq) == 0)
        {
            // Storing in the sequence.
            array_push($seq, $b * $i);
  
            sort($seq);
            $k--;
        }
    }
 
    return $seq[$n - 1];
}
 
// Driver Code
$a = 3;
$b = 5;
$n = 5;
echo nthElement($a, $b, $n);
 
// This code is contributed by mits
?>

Output:  

10

Method 2: (Efficient Approach) 
The idea is to use the fact that common multiple of a and b are removed using LCM(a, b). Let f(a, b, x) be a function which calculates the count of number that are less than x and multiples of a and b. Now, using the inclusion-exclusion principle we can say, 
 



f(a, b, x) :  Count of number that are less than x
              and multiples of a and b

f(a, b, x) = (x/a) + (x/b) - (x/lcm(a, b))
where (x/a) define number of multiples of a
(x/b) define number of multiple of b
(x/lcm(a, b)) define the number of common multiples 
of a and b.

Observe, a and b are constant. As x increases, f(a, b, x) will also increase. Therefore we can apply Binary Search to find the minimum value of x such that f(a, b, x) >= n. The lower bound of the function is the required answer.
The upper bound for n-th term would be min(a, b)*n. Note that we get the largest value n-th term when there are no common elements in multiples of a and b.
Below are the implementations of above approach: 
 

C++




// C++ program to find n-th number in thes
// sorted list of multiples of two numbers.
#include<bits/stdc++.h>
using namespace std;
 
// Return the Nth number in the sorted
// list of multiples of two numbers.
int nthElement(int a, int b, int n)
{
    // Finding LCM of a and b.
    int lcm = (a * b)/__gcd(a,b);
 
    // Binary Search.
    int l = 1, r = min(a, b)*n;
    while (l <= r)
    {
        // Finding the middle element.
        int mid = (l + r)>>1;
 
        // count of number that are less than
        // mid and multiples of a and b
        int val = mid/a + mid/b - mid/lcm;
 
        if (val == n)
            return max((mid/a)*a, (mid/b)*b);
 
        if (val < n)
            l = mid + 1;
        else
            r = mid - 1;
    }
}
 
// Driven Program
int main()
{
    int a = 5, b = 3, n = 5;
    cout << nthElement(a, b, n) << endl;
    return 0;
}

Java




// Java program to find n-th number in the
// sorted list of multiples of two numbers.
import java.io.*;
 
public class GFG{
     
// Recursive function to return
// gcd of a and b
static int __gcd(int a, int b)
{
    // Everything divides 0
    if (a == 0 || b == 0)
    return 0;
 
    // base case
    if (a == b)
        return a;
     
    // a is greater
    if (a > b)
        return __gcd(a - b, b);
        return __gcd(a, b - a);
}
 
// Return the Nth number in the sorted
// list of multiples of two numbers.
static int nthElement(int a, int b, int n)
{
    // Finding LCM of a and b.
    int lcm = (a * b) / __gcd(a, b);
 
    // Binary Search.
    int l = 1, r = Math.min(a, b) * n;
    while (l <= r)
    {
        // Finding the middle element.
        int mid = (l + r) >> 1;
 
        // count of number that are less than
        // mid and multiples of a and b
        int val = mid / a + mid / b -
                  mid / lcm;
 
        if (val == n)
            return Math.max((mid / a) * a,
                            (mid / b) * b);
 
        if (val < n)
            l = mid + 1;
        else
            r = mid - 1;
    }
    return 0;
}
 
// Driver Code
static public void main (String[] args)
{
    int a = 5, b = 3, n = 5;
    System.out.println(nthElement(a, b, n));
}
}
 
// This code is contributed by vt_m.

Python 3




# Python 3 program to find n-th number
# in thes sorted list of multiples of
# two numbers.
import math
 
# Return the Nth number in the sorted
# list of multiples of two numbers.
def nthElement(a, b, n):
 
    # Finding LCM of a and b.
    lcm = (a * b) / int(math.gcd(a, b))
 
    # Binary Search.
    l = 1
    r = min(a, b) * n
    while (l <= r):
     
        # Finding the middle element.
        mid = (l + r) >> 1
 
        # count of number that are less
        # than mid and multiples of a
        # and b
        val = (int(mid / a) + int(mid / b)
                         - int(mid / lcm))
 
        if (val == n):
            return int( max(int(mid / a) * a,
                          int(mid / b) * b) )
 
        if (val < n):
            l = mid + 1
        else:
            r = mid - 1
     
# Driven Program
a = 5
b = 3
n = 5
print(nthElement(a, b, n))
 
# This code is contributed by Smitha.

C#




// C# program to find n-th number in thes
// sorted list of multiples of two numbers.
using System;
 
public class GFG{
     
// Recursive function to return
// gcd of a and b
static int __gcd(int a, int b)
{
    // Everything divides 0
    if (a == 0 || b == 0)
    return 0;
 
    // base case
    if (a == b)
        return a;
     
    // a is greater
    if (a > b)
        return __gcd(a - b, b);
        return __gcd(a, b - a);
}
 
// Return the Nth number in the sorted
// list of multiples of two numbers.
static int nthElement(int a, int b, int n)
{
    // Finding LCM of a and b.
    int lcm = (a * b) / __gcd(a, b);
 
    // Binary Search.
    int l = 1, r = Math.Min(a, b) * n;
    while (l <= r)
    {
        // Finding the middle element.
        int mid = (l + r) >> 1;
 
        // count of number that are less than
        // mid and multiples of a and b
        int val = mid / a + mid / b -
                  mid / lcm;
 
        if (val == n)
            return Math.Max((mid / a) * a,
                            (mid / b) * b);
 
        if (val < n)
            l = mid + 1;
        else
            r = mid - 1;
    }
    return 0;
}
 
// Driver Code
    static public void Main (String []args)
    {
        int a = 5, b = 3, n = 5;
            Console.WriteLine(nthElement(a, b, n));
    }
}
 
// This code is contributed by vt_m.

PHP




<?php
// PHP program to find n-th number
// in a sorted list of multiples
// of two numbers.
 
// function to calculate gcd
function gcd($a, $b)
{
    return ($a % $b) ?
        gcd($b, $a % $b) : $b;
}
 
// Return the Nth number in the sorted
// list of multiples of two numbers.
function nthElement($a, $b, $n)
{
    // Finding LCM of a and b.
    $lcm = (int)(($a * $b) / gcd($a, $b));
 
    // Binary Search.
    $l = 1;
    $r = min($a, $b) * $n;
    while ($l <= $r)
    {
        // Finding the middle element.
        $mid = ($l + $r) >> 1;
 
        // count of number that are
        // less than mid and multiples
        // of a and b
        $val = (int)($mid / $a) +
               (int)($mid / $b) -
               (int)($mid / $lcm);
 
        if ($val == $n)
            return max((int)(($mid / $a)) * $a,
                       (int)(($mid / $b)) * $b);
 
        if ($val < $n)
            $l = $mid + 1;
        else
            $r = $mid - 1;
    }
}
 
// Driver code
$a = 5;
$b = 3;
$n = 5;
echo nthElement($a, $b, $n);
 
// This code is contributed by mits
?>

Javascript




<script>
 
// Javascript program to find n-th number in the
// sorted list of multiples of two numbers.
     
// Recursive function to return
// gcd of a and b
function __gcd(a , b)
{
    // Everything divides 0
    if (a == 0 || b == 0)
    return 0;
 
    // base case
    if (a == b)
        return a;
     
    // a is greater
    if (a > b)
        return __gcd(a - b, b);
        return __gcd(a, b - a);
}
 
// Return the Nth number in the sorted
// list of multiples of two numbers.
function nthElement(a , b , n)
{
    // Finding LCM of a and b.
    var lcm = parseInt((a * b) / __gcd(a, b));
 
    // Binary Search.
    var l = 1, r = Math.min(a, b) * n;
    while (l <= r)
    {
        // Finding the middle element.
        var mid = (l + r) >> 1;
 
        // count of number that are less than
        // mid and multiples of a and b
        var val = parseInt(mid / a) + parseInt(mid / b) -
                  parseInt(mid / lcm);
 
        if (val == n)
            return Math.max(parseInt(mid / a) * a,
                            parseInt(mid / b) * b);
 
        if (val < n)
            l = mid + 1;
        else
            r = mid - 1;
    }
    return 0;
}
 
// Driver Code
var a = 5, b = 3, n = 5;
document.write(nthElement(a, b, n));
 
 
// This code contributed by shikhasingrajput
 
</script>

Output:  

10

This article is contributed by Anuj Chauhan. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
 

Attention reader! Don’t stop learning now. Get hold of all the important mathematical concepts for competitive programming with the Essential Maths for CP Course at a student-friendly price. To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.

My Personal Notes arrow_drop_up
Recommended Articles
Page :