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k-th number in the Odd-Even sequence
  • Difficulty Level : Medium
  • Last Updated : 20 Nov, 2019

Given two numbers n and k, find the k-th number in the Odd-Even sequence made of n. The Odd-Even sequence contains first contains all odd numbers from 1 to n then all even numbers in set 1 to n.

Examples :
Input :  n = 5, k = 3
Output : 5
In this example, the Odd-Even  is
{1, 3, 5, 2, 4}. 
The third number in sequence is 5.

Naive Approach :

The first approach is simply make a Odd-Even sequence and then find k-th element in it.

C++

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// CPP program to find k-th 
// element in the Odd-Even 
// sequence.
#include <bits/stdc++.h>
using namespace std;
  
int findK(int n, int k)
{
    vector<long> a;
      
    // insert all the odd 
    // numbers from 1 to n. 
    for (int i = 1; i < n; i++) 
        if (i % 2 == 1)
            a.push_back(i);
      
    // insert all the even 
    // numbers from 1 to n.
    for (int i = 1; i < n; i++) 
        if (i % 2 == 0)
            a.push_back(i);
      
    return (a[k - 1]);
}
  
// Driver code
int main()
{
    long n = 10, k = 3;
    cout << findK(n, k) << endl;
    return 0;
}

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Java

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// Java program to find k-th 
// element in the Odd-Even 
// sequence. 
import java.util.*;
  
class GFG{
static int findK(int n, int k) 
    ArrayList<Integer> a = new ArrayList<Integer>(n);
      
    // insert all the odd 
    // numbers from 1 to n. 
    for (int i = 1; i < n; i++) 
        if (i % 2 == 1
            a.add(i); 
      
    // insert all the even 
    // numbers from 1 to n. 
    for (int i = 1; i < n; i++) 
        if (i % 2 == 0
            a.add(i); 
      
    return (a.get(k - 1)); 
  
// Driver code 
public static void main(String[] args) 
    int n = 10, k = 3
    System.out.println(findK(n, k)); 
}
// This code is contributed by mits

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Python3

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# Python3 code to find 
# k-th element in the
# Odd-Even sequence.
  
def findK (n, k ):
    a = list()
      
    # insert all the odd 
    # numbers from 1 to n.
    i = 1
    while i < n:
        a.append(i)
        i = i + 2
      
    # insert all the even 
    # numbers from 1 to n.
    i = 2
    while i < n:
        a.append(i)
        i = i + 2
          
    return (a[k - 1])
  
# Driver code
n = 10
k = 3
print(findK(n, k))
  
# This code is contributed 
# by "Sharad_Bhardwaj".

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C#

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// C# program to find k-th 
// element in the Odd-Even 
// sequence. 
using System; 
using System.Collections; 
  
class GFG{
static int findK(int n, int k) 
    ArrayList a = new ArrayList(n);
      
    // insert all the odd 
    // numbers from 1 to n. 
    for (int i = 1; i < n; i++) 
        if (i % 2 == 1) 
            a.Add(i); 
      
    // insert all the even 
    // numbers from 1 to n. 
    for (int i = 1; i < n; i++) 
        if (i % 2 == 0) 
            a.Add(i); 
      
    return (int)(a[k - 1]); 
  
// Driver code 
static void Main() 
    int n = 10, k = 3; 
    Console.WriteLine(findK(n, k)); 
}
// This code is contributed by mits

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PHP

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<?php
// PHP program to find k-th 
// element in the Odd-Even 
// sequence.
  
function findK($n, $k)
{
    $a;
    $index = 0;
      
    // insert all the odd 
    // numbers from 1 to n. 
    for ($i = 1; $i < $n; $i++) 
        if ($i % 2 == 1)
            $a[$index++] = $i;
      
    // insert all the even 
    // numbers from 1 to n.
    for ($i = 1; $i < $n; $i++) 
        if ($i % 2 == 0)
            $a[$index++] = $i;
      
    return ($a[$k - 1]);
}
  
// Driver code
$n = 10;
$k = 3;
echo findK($n, $k);
  
// This code is contributed by mits.
?>

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Output :

 5

The time complexity of the above approach is O(n), for an O(1) approach read the approach discussed here.

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