Rearrange array in alternating positive & negative items with O(1) extra space | Set 2

Given an array of positive and negative numbers, arrange them in an alternate fashion such that every positive number is followed by negative and vice-versa. Order of elements in output doesn’t matter. Extra positive or negative elements should be moved to end.

Examples:

Input :
arr[] = {-2, 3, 4, -1}
Output :
arr[] = {-2, 3, -1, 4} OR {-1, 3, -2, 4} OR ..

Input :
arr[] = {-2, 3, 1}
Output :
arr[] = {-2, 3, 1} OR {-2, 1, 3} 

Input : 
arr[] = {-5, 3, 4, 5, -6, -2, 8, 9, -1, -4}
Output :
arr[] = {-5, 3, -2, 5, -6, 4, -4, 9, -1, 8} 
        OR ..

We strongly recommend you to minimize your browser and try this yourself first.

We have already discussed a O(n2) solution that maintains the order of appearance in the array here. If we are allowed to change order of appearance, we can solve this problem in O(n) time and O(1) space.

The idea is to process the array and shift all negative values to the end in O(n) time. After all negative values are shifted to the end, we can easily rearrange array in alternating positive & negative items. We basically swap next positive element at even position from next negative element in this step.

Following is the implementation of above idea.

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

// C++ program to rearrange array in alternating
// positive & negative items with O(1) extra space
#include <bits/stdc++.h>
using namespace std;
  
// Function to rearrange positive and negative
// integers in alternate fashion. The below
// solution doesn't maintain original order of
// elements
void rearrange(int arr[], int n)
{
    int i = -1, j = n;
  
    // shift all negative values to the end
    while (i < j)
    {
        while (arr[++i] > 0);
        while (arr[--j] < 0);
        if (i < j)
            swap(arr[i], arr[j]);
    }
  
    // i has index of leftmost negative element
    if (i == 0 || i == n)
        return;
  
    // start with first positive element at index 0
  
    // Rearrange array in alternating positive &
    // negative items
    int k = 0;
    while (k < n && i < n)
    {
        // swap next positive element at even position
        // from next negative element.
        swap(arr[k], arr[i]);
        i = i + 1;
        k = k + 2;
    }
}
  
// Utility function to print an array
void printArray(int arr[], int n)
{
    for (int i = 0; i < n; i++)
      cout << arr[i] << " ";
    cout << endl;
}
  
// Driver code
int main()
{
    int arr[] = {2, 3, -4, -1, 6, -9};
  
    int n = sizeof(arr)/sizeof(arr[0]);
  
    cout << "Given array is \n";
    printArray(arr, n);
  
    rearrange(arr, n);
  
    cout << "Rearranged array is \n";
    printArray(arr, n);
  
    return 0;
}

chevron_right


Java

filter_none

edit
close

play_arrow

link
brightness_4
code

//Java program to rearrange array in alternating 
// positive & negative items with O(1) extra space 
class GFG {
  
  
// Function to rearrange positive and negative 
// integers in alternate fashion. The below 
// solution doesn't maintain original order of 
// elements 
static void rearrange(int arr[], int n) 
    int i = -1, j = n; 
  
    // shift all negative values to the end 
    while (i < j) 
    
        while (arr[++i] > 0); 
        while (arr[--j] < 0); 
        if (i < j) 
            swap(arr, i,j); 
    
  
    // i has index of leftmost negative element 
    if (i == 0 || i == n) 
        return
  
    // start with first positive element at index 0 
  
    // Rearrange array in alternating positive & 
    // negative items 
    int k = 0
    while (k < n && i < n) 
    
        // swap next positive element at even position 
        // from next negative element. 
        swap(arr,k,i); 
        i = i + 1
        k = k + 2
    
  
// Utility function to print an array 
static void printArray(int arr[], int n) 
    for (int i = 0; i < n; i++) 
           System.out.print(arr[i] + " "); 
       System.out.println("");
  
  
static void swap(int arr[], int index1, int index2){
    int c = arr[index1];
    arr[index1]=arr[index2];
    arr[index2]=c;
  
  
// Driver code 
    public static void main(String[] args) {
        int arr[] = {2, 3, -4, -1, 6, -9}; 
  
    int n = arr.length; 
  
    System.out.println("Given array is "); 
    printArray(arr, n); 
  
    rearrange(arr, n); 
  
    System.out.println("Rearranged array is "); 
    printArray(arr, n); 
    }
  
  
// This code is contributed by 29AjayKumar

chevron_right


Python3

filter_none

edit
close

play_arrow

link
brightness_4
code

# Python3 program to rearrange array 
# in alternating positive & negative 
# items with O(1) extra space 
  
# Function to rearrange positive and 
# negative integers in alternate fashion. 
# The below solution does not maintain 
# original order of elements 
def rearrange(arr, n): 
    i = 0
    j = n - 1
  
    # shift all negative values 
    # to the end 
    while (i < j): 
        while (arr[i] > 0):
            i += 1
        while (arr[j] < 0):
            j -= 1
        if (i < j): 
            temp = arr[i] 
            arr[i] = arr[j] 
            arr[j] = temp 
              
    # i has index of leftmost 
    # negative element 
    if (i == 0 or i == n): 
        return 0
  
    # start with first positive element 
    # at index 0 
  
    # Rearrange array in alternating 
    # positive & negative items 
    k = 0
    while (k < n and i < n): 
          
        # swap next positive element at even 
        # position from next negative element. 
        temp = arr[k] 
        arr[k] = arr[i] 
        arr[i] = temp 
        i = i + 1
        k = k + 2
  
# Utility function to print an array 
def printArray(arr, n): 
    for i in range(n): 
        print(arr[i], end = " "
    print("\n"
  
# Driver code 
arr = [2, 3, -4, -1, 6, -9
  
n = len(arr) 
  
print( "Given array is"
printArray(arr, n) 
  
rearrange(arr, n) 
  
print( "Rearranged array is"
printArray(arr, n) 
  
# This code is contributed 
# Princi Singh

chevron_right


C#

filter_none

edit
close

play_arrow

link
brightness_4
code

// C# program to rearrange array 
// in alternating positive & negative 
// items with O(1) extra space 
using System;
  
class GFG
{
  
// Function to rearrange positive and 
// negative integers in alternate fashion. 
// The below solution doesn't maintain 
// original order of elements 
static void rearrange(int []arr, int n) 
    int i = -1, j = n; 
  
    // shift all negative values 
    // to the end 
    while (i < j) 
    
        while (arr[++i] > 0); 
        while (arr[--j] < 0); 
        if (i < j) 
            swap(arr, i,j); 
    
  
    // i has index of leftmost 
    // negative element 
    if (i == 0 || i == n) 
        return
  
    // start with first positive 
    // element at index 0 
  
    // Rearrange array in alternating 
    // positive & negative items 
    int k = 0; 
    while (k < n && i < n) 
    
        // swap next positive element 
        // at even position from next 
        // negative element. 
        swap(arr, k, i); 
        i = i + 1; 
        k = k + 2; 
    
  
// Utility function to print an array 
static void printArray(int []arr, int n) 
    for (int i = 0; i < n; i++) 
        Console.Write(arr[i] + " "); 
    Console.WriteLine(""); 
  
static void swap(int []arr, int index1,
                            int index2)
    int c = arr[index1]; 
    arr[index1] = arr[index2]; 
    arr[index2] = c; 
  
// Driver code 
public static void Main() 
    int []arr = {2, 3, -4, -1, 6, -9}; 
  
    int n = arr.Length; 
      
    Console.WriteLine("Given array is "); 
    printArray(arr, n); 
      
    rearrange(arr, n); 
      
    Console.WriteLine("Rearranged array is "); 
    printArray(arr, n); 
  
// This code is contributed 
// by 29AjayKumar

chevron_right


PHP

filter_none

edit
close

play_arrow

link
brightness_4
code

<?php 
// PHP program to rearrange array 
// in alternating positive & negative
// items with O(1) extra space
  
// Function to rearrange positive and 
// negative integers in alternate fashion.
// The below solution doesn't maintain 
// original order of elements
function rearrange(&$arr, $n)
{
    $i = -1;
    $j = $n;
  
    // shift all negative values
    // to the end
    while ($i < $j)
    {
        while ($arr[++$i] > 0);
        while ($arr[--$j] < 0);
        if ($i < $j)
        {
            $temp = $arr[$i];
            $arr[$i] = $arr[$j];
            $arr[$j] = $temp;
        }         
    }
  
    // i has index of leftmost
    // negative element
    if ($i == 0 || $i == $n)
        return;
  
    // start with first positive element
    // at index 0
  
    // Rearrange array in alternating
    // positive & negative items
    $k = 0;
    while ($k < $n && $i < $n)
    {
        // swap next positive element at even 
        // position from next negative element.
        $temp = $arr[$k];
        $arr[$k] = $arr[$i];
        $arr[$i] = $temp;
        $i = $i + 1;
        $k = $k + 2;
    }
}
  
// Utility function to print an array
function printArray(&$arr, $n)
{
    for ($i = 0; $i < $n; $i++)
    echo $arr[$i] . " ";
    echo "\n";
}
  
// Driver code
$arr = array(2, 3, -4, -1, 6, -9);
  
$n = sizeof($arr);
  
echo "Given array is \n";
printArray($arr, $n);
  
rearrange($arr, $n);
  
echo "Rearranged array is \n";
printArray($arr, $n);
  
// This code is contributed 
// by ChitraNayal
?>

chevron_right



Output:

Given array is 
2 3 -4 -1 6 -9 
Rearranged array is 
-1 3 -4 2 -9 6 

This article is contributed by Aditya Goel. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above



My Personal Notes arrow_drop_up



Article Tags :
Practice Tags :


2


Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.