Print array elements in alternatively increasing and decreasing order

Given an array of N elements. The task is to print the array elements in such a way that first two elements are in increasing order, next 3 in decreasing order, next 4 in increasing order and so on.

Examples:

Input : arr = {2, 6, 2, 4, 0, 1, 4, 8, 2, 0, 0, 5}
Output : 0 0 8 6 5 0 1 2 2 4 4 2 2 2

Input : arr = {1, 2, 3, 4, 5, 6}
Output : 1 2 6 5 4 3

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Source :Oracle Interview experience set 52

The idea is to use 2 pointer technique. First sort the array in increasing order and maintain two pointers $l$ and $r$ where $l$ is to print the array in increasing order and $r$ to print the array in decreasing order. Keep a variable $K$ to specify the number of elements to be printed in an iteration and a variable flag to switch between printing in increasing order and decreasing order alternatively.

Below is the implementation of above approach:

C++

// C++ program to print array elements in 
// alternative increasing and decreasing 
// order 
  
#include <bits/stdc++.h> 
using namespace std; 
  
// Function to print array elements in 
// alternative increasing and decreasing 
// order 
void printArray(int arr[], int n) 
{ 
    // First sort the array in increasing order 
    sort(arr, arr + n); 
  
    int l = 0, r = n - 1, flag = 0, i; 
  
    // start with 2 elements in 
    // increasing order 
    int k = 2; 
  
    // till all the elements are not printed 
    while (l <= r) { 
        // printing the elements in 
        // increasing order 
        if (flag == 0) { 
            for (i = l; i < l + k && i <= r; i++) 
                cout << arr[i] << " "; 
  
            flag = 1; 
            l = i; 
        } 
        else // printing the elements in 
        // decreasing order 
        { 
            for (i = r; i > r - k && i >= l; i--) 
                cout << arr[i] << " "; 
  
            flag = 0; 
            r = i; 
        } 
  
        // increasing the number of elements 
        // to printed in next iteration 
        k++; 
    } 
} 
  
// Driver Code 
int main() 
{ 
    int n = 6; 
  
    int arr[] = { 1, 2, 3, 4, 5, 6 }; 
  
    printArray(arr, n); 
  
    return 0; 
} 

Java

// Java program to print array elements in 
// alternative increasing and decreasing 
// order 
import java.util.*; 
class Solution 
{ 
   
// Function to print array elements in 
// alternative increasing and decreasing 
// order 
static void printArray(int arr[], int n) 
{ 
    // First sort the array in increasing order 
    Arrays.sort(arr); 
   
    int l = 0, r = n - 1, flag = 0, i; 
   
    // start with 2 elements in 
    // increasing order 
    int k = 2; 
   
    // till all the elements are not printed 
    while (l <= r) { 
        // printing the elements in 
        // increasing order 
        if (flag == 0) { 
            for (i = l; i < l + k && i <= r; i++) 
                System.out.print(arr[i] + " "); 
   
            flag = 1; 
            l = i; 
        } 
        else // printing the elements in 
        // decreasing order 
        { 
            for (i = r; i > r - k && i >= l; i--) 
                System.out.print(arr[i] + " "); 
   
            flag = 0; 
            r = i; 
        } 
   
        // increasing the number of elements 
        // to printed in next iteration 
        k++; 
    } 
} 
   
// Driver Code 
public static void main(String args[]) 
{ 
    int n = 6; 
   
    int arr[] = { 1, 2, 3, 4, 5, 6 }; 
   
    printArray(arr, n); 
   
  
} 
  
} 
//contributed by Arnab Kundu 

Python 3

# Python 3 program to print array elements  
# in alternative increasing and decreasing 
# order 
  
# Function to print array elements in 
# alternative increasing and decreasing 
# order 
def printArray(arr, n): 
  
    # First sort the array in 
    # increasing order 
    arr.sort() 
  
    l = 0
    r = n - 1
    flag = 0
      
    # start with 2 elements in 
    # increasing order 
    k = 2
  
    # till all the elements are not printed 
    while (l <= r) : 
          
        # printing the elements in 
        # increasing order 
        if (flag == 0): 
            i = l 
            while i < l + k and i <= r: 
                print(arr[i], end = " ") 
                i += 1
  
            flag = 1
            l = i 
          
        else:     # printing the elements in 
                 # decreasing order 
            i = r 
            while i > r - k and i >= l: 
                print(arr[i], end = " ") 
                i -= 1
  
            flag = 0
            r = i 
  
        # increasing the number of elements 
        # to printed in next iteration 
        k += 1
  
# Driver Code 
if __name__ == "__main__": 
      
    n = 6
    arr = [ 1, 2, 3, 4, 5, 6 ]  
    printArray(arr, n) 
  
# This code is contributed by ita_c 

C#

// C# program to print array elements in  
// alternative increasing and decreasing  
// order  
using System; 
   
class GFG{ 
       
// Function to print array elements in  
// alternative increasing and decreasing  
// order  
static void printArray(int []arr, int n)  
{  
      
    // First sort the array 
    // in increasing order  
    Array.Sort(arr);  
   
    int l = 0, r = n - 1, flag = 0, i;  
   
    // start with 2 elements in  
    // increasing order  
    int k = 2;  
   
    // till all the elements 
    // are not printed  
    while (l <= r) {  
          
        // printing the elements in  
        // increasing order  
        if (flag == 0) { 
              
            for (i = l; i < l + k && i <= r; i++)  
                    Console.Write(arr[i] + " ");  
   
            flag = 1;  
            l = i;  
        }  
        else 
          
        // printing the elements in  
        // decreasing order  
        {  
            for (i = r; i > r - k && i >= l; i--)  
                Console.Write(arr[i] + " ");  
   
            flag = 0;  
            r = i;  
        }  
   
        // increasing the number of elements  
        // to printed in next iteration  
        k++;  
    }  
}  
   
// Driver Code  
static public void Main () 
{ 
           
    int n = 6;  
    int []arr = { 1, 2, 3, 4, 5, 6 };  
    printArray(arr, n);  
  
}  
}  
  
// This code is contributed by Sach_Code 

PHP

<?php 
// PHP program to print array elements in  
// alternative increasing and decreasing  
// order  
  
// Function to print array elements in  
// alternative increasing and decreasing  
// order  
function printArray($arr, $n)  
{  
    // First sort the array in  
    // increasing order  
    sort($arr);  
  
    $l = 0; 
    $r = $n - 1; 
    $flag = 0; 
  
    // start with 2 elements in  
    // increasing order  
    $k = 2;  
  
    // till all the elements are 
    // not printed  
    while ($l <= $r)  
    {  
        // printing the elements in  
        // increasing order  
        if ($flag == 0)  
        {  
            for ($i = $l;  
                 $i < $l + $k &&  
                 $i <= $r; $i++)  
                echo $arr[$i] , " ";  
  
            $flag = 1;  
            $l = $i;  
        }  
        else // printing the elements in  
             // decreasing order  
        {  
            for ($i = $r; 
                 $i > $r - $k &&  
                 $i >= $l; $i--)  
                echo $arr[$i] , " ";  
  
            $flag = 0;  
            $r = $i;  
        }  
  
        // increasing the number of elements  
        // to printed in next iteration  
        $k++;  
    }  
}  
  
// Driver Code  
$n = 6;  
$arr = array( 1, 2, 3, 4, 5, 6 );  
  
printArray($arr, $n);  
  
// This code is contributed by jit_t 
?> 

Output:

1 2 6 5 4 3

Time Complexity : O(nlogn)

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My Personal Notes

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

Python 3

C#

PHP

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