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

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 maintaining the order of appearance.
Number of positive and negative numbers need not be equal. If there are more positive numbers they appear at the end of the array. If there are more negative numbers, they too appear in the end of the array.

Examples :

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

Input:  arr[] = {-5, -2, 5, 2, 4, 7, 1, 8, 0, -8}
output: arr[] = {-5, 5, -2, 2, -8, 4, 7, 1, 8, 0} 

This question has been asked at many places (See this and this)



The above problem can be easily solved if O(n) extra space is allowed. It becomes interesting due to the limitations that O(1) extra space and order of appearances.
The idea is to process array from left to right. While processing, find the first out of place element in the remaining unprocessed array. An element is out of place if it is negative and at odd index, or it is positive and at even index. Once we find an out of place element, we find the first element after it with opposite sign. We right rotate the subarray between these two elements (including these two).

Following is the implementation of above idea.

C++

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/*  C++ program to rearrange positive and negative integers in alternate
    fashion while keeping the order of positive and negative numbers. */
#include <iostream>
#include <assert.h>
using namespace std;
  
// Utility function to right rotate all elements between [outofplace, cur]
void rightrotate(int arr[], int n, int outofplace, int cur)
{
    char tmp = arr[cur];
    for (int i = cur; i > outofplace; i--)
        arr[i] = arr[i-1];
    arr[outofplace] = tmp;
}
  
void rearrange(int arr[], int n)
{
    int outofplace = -1;
  
    for (int index = 0; index < n; index ++)
    {
        if (outofplace >= 0)
        {
            // find the item which must be moved into the out-of-place
            // entry if out-of-place entry is positive and current
            // entry is negative OR if out-of-place entry is negative
            // and current entry is negative then right rotate
            //
            // [...-3, -4, -5, 6...] -->   [...6, -3, -4, -5...]
            //      ^                          ^
            //      |                          |
            //     outofplace      -->      outofplace
            //
            if (((arr[index] >= 0) && (arr[outofplace] < 0))
                || ((arr[index] < 0) && (arr[outofplace] >= 0)))
            {
                rightrotate(arr, n, outofplace, index);
  
                // the new out-of-place entry is now 2 steps ahead
                if (index - outofplace >= 2)
                    outofplace = outofplace + 2;
                else
                    outofplace = -1;
            }
        }
  
  
        // if no entry has been flagged out-of-place
        if (outofplace == -1)
        {
            // check if current entry is out-of-place
            if (((arr[index] >= 0) && (!(index & 0x01)))
                || ((arr[index] < 0) && (index & 0x01)))
            {
                outofplace = index;
            }
        }
    }
}
  
// A utility function to print an array 'arr[]' of size 'n'
void printArray(int arr[], int n)
{
    for (int i = 0; i < n; i++)
      cout << arr[i] << " ";
    cout << endl;
}
  
// Driver program to test abive function
int main()
{
    //int arr[n] = {-5, 3, 4, 5, -6, -2, 8, 9, -1, -4};
    //int arr[] = {-5, -3, -4, -5, -6, 2 , 8, 9, 1 , 4};
    //int arr[] = {5, 3, 4, 2, 1, -2 , -8, -9, -1 , -4};
    //int arr[] = {-5, 3, -4, -7, -1, -2 , -8, -9, 1 , -4};
    int arr[] = {-5, -2, 5, 2, 4, 7, 1, 8, 0, -8};
    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;
}

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Java

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class RearrangeArray 
{
    // Utility function to right rotate all elements 
    // between [outofplace, cur]
    void rightrotate(int arr[], int n, int outofplace, int cur) 
    {
        int tmp = arr[cur];
        for (int i = cur; i > outofplace; i--)
            arr[i] = arr[i - 1];
        arr[outofplace] = tmp;
    }
  
    void rearrange(int arr[], int n) 
    {
        int outofplace = -1;
  
        for (int index = 0; index < n; index++) 
        {
            if (outofplace >= 0
            {
                // find the item which must be moved into the out-of-place
                // entry if out-of-place entry is positive and current
                // entry is negative OR if out-of-place entry is negative
                // and current entry is negative then right rotate
                //
                // [...-3, -4, -5, 6...] -->   [...6, -3, -4, -5...]
                //      ^                          ^
                //      |                          |
                //     outofplace      -->      outofplace
                //
                if (((arr[index] >= 0) && (arr[outofplace] < 0))
                        || ((arr[index] < 0) && (arr[outofplace] >= 0))) 
                {
                    rightrotate(arr, n, outofplace, index);
  
                    // the new out-of-place entry is now 2 steps ahead
                    if (index - outofplace > 2
                        outofplace = outofplace + 2;
                    else
                        outofplace = -1;
                }
            }
  
            // if no entry has been flagged out-of-place
            if (outofplace == -1
            {
                // check if current entry is out-of-place
                if (((arr[index] >= 0) && ((index & 0x01)==0))
                        || ((arr[index] < 0) && (index & 0x01)==1))
                    outofplace = index;
            }
        }
    }
   
    // A utility function to print an array 'arr[]' of size 'n'
    void printArray(int arr[], int n) 
    {
        for (int i = 0; i < n; i++)
            System.out.print(arr[i] + " ");
        System.out.println("");
    }
  
    public static void main(String[] args) 
    {
        RearrangeArray rearrange = new RearrangeArray();
        //int arr[n] = {-5, 3, 4, 5, -6, -2, 8, 9, -1, -4};
        //int arr[] = {-5, -3, -4, -5, -6, 2 , 8, 9, 1 , 4};
        //int arr[] = {5, 3, 4, 2, 1, -2 , -8, -9, -1 , -4};
        //int arr[] = {-5, 3, -4, -7, -1, -2 , -8, -9, 1 , -4};
        int arr[] = {-5, -2, 5, 2, 4, 7, 1, 8, 0, -8};
        int n = arr.length;
  
        System.out.println("Given array is ");
        rearrange.printArray(arr, n);
  
        rearrange.rearrange(arr, n);
  
        System.out.println("RearrangeD array is ");
        rearrange.printArray(arr, n);
    }
}
  
// This code has been contributed by Mayank Jaiswal

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Python3

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# Python3 program to rearrange 
# positive and negative integers 
# in alternate fashion and 
# maintaining the order of positive 
# and negative numbers
  
# rotates the array to right by once
# from index 'outOfPlace to cur'
def rightRotate(arr, n, outOfPlace, cur):
    temp = arr[cur]
    for i in range(cur, outOfPlace, -1):
        arr[i] = arr[i - 1]
    arr[outOfPlace] = temp
    return arr
  
def rearrange(arr, n):
    outOfPlace = -1
    for index in range(n):
        if(outOfPlace >= 0):
  
            # if element at outOfPlace place in 
            # negative and if element at index
            # is positive we can rotate the 
            # array to right or if element
            # at outOfPlace place in positive and
            # if element at index is negative we 
            # can rotate the array to right
            if((arr[index] >= 0 and arr[outOfPlace] < 0) or 
               (arr[index] < 0 and arr[outOfPlace] >= 0)):
                arr = rightRotate(arr, n, outOfPlace, index)
                if(index-outOfPlace > 2):
                    outOfPlace += 2
                else:
                    outOfPlace =- 1
                      
        if(outOfPlace == -1):
              
            # conditions for A[index] to 
            # be in out of place
            if((arr[index] >= 0 and index % 2 == 0) or 
               (arr[index] < 0 and index % 2 == 1)):
                outOfPlace = index
    return arr
  
# Driver Code
arr = [-5, -2, 5, 2, 4
        7, 1, 8, 0, -8]
  
print("Given Array is:")
print(arr)
  
print("\nRearranged array is:")
print(rearrange(arr, len(arr)))
  
# This code is contributed
# by Charan Sai

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C#

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// Rearrange array in alternating positive
// & negative items with O(1) extra space
using System;
  
class GFG
{
    // Utility function to right rotate
    // all elements between [outofplace, cur]
    static void rightrotate(int []arr, int n,
                            int outofplace, int cur) 
    {
        int tmp = arr[cur];
        for (int i = cur; i > outofplace; i--)
            arr[i] = arr[i - 1];
        arr[outofplace] = tmp;
    }
  
    static void rearrange(int []arr, int n) 
    {
        int outofplace = -1;
  
        for (int index = 0; index < n; index++) 
        {
            if (outofplace >= 0) 
            {
                // find the item which must be moved
                // into the out-of-place entry if out-of-
                // place entry is positive and current
                // entry is negative OR if out-of-place
                // entry is negative and current entry 
                // is negative then right rotate
                // [...-3, -4, -5, 6...] --> [...6, -3, -4, -5...]
                //     ^                         ^
                //     |                         |
                //     outofplace     -->     outofplace
                //
                if (((arr[index] >= 0) &&
                     (arr[outofplace] < 0)) ||
                     ((arr[index] < 0) &&
                     (arr[outofplace] >= 0))) 
                {
                    rightrotate(arr, n, outofplace, index);
  
                    // the new out-of-place entry 
                    // is now 2 steps ahead
                    if (index - outofplace > 2) 
                        outofplace = outofplace + 2;
                    else
                        outofplace = -1;
                }
            }
  
            // if no entry has been flagged out-of-place
            if (outofplace == -1) 
            {
                // check if current entry is out-of-place
                if (((arr[index] >= 0) &&
                    ((index & 0x01)==0)) || 
                    ((arr[index] < 0) &&
                    (index & 0x01)==1))
                    outofplace = index;
            }
        }
    }
  
    // A utility function to print an
    // array 'arr[]' of size 'n'
    static void printArray(int []arr, int n) 
    {
        for (int i = 0; i < n; i++)
            Console.Write(arr[i] + " ");
        Console.WriteLine("");
    }
  
    // Driver code
    public static void Main() 
    {
        int []arr = {-5, -2, 5, 2, 4,
                      7, 1, 8, 0, -8};
        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 Sam007

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PHP

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<?php 
// PHP program to rearrange positive and 
// negative integers in alternate fashion 
// while keeping the order of positive 
// and negative numbers. 
  
// Utility function to right rotate all
// elements between [outofplace, cur]
function rightrotate(&$arr, $n
                      $outofplace, $cur)
{
    $tmp = $arr[$cur];
    for ($i = $cur; $i > $outofplace; $i--)
        $arr[$i] = $arr[$i - 1];
    $arr[$outofplace] = $tmp;
}
  
function rearrange(&$arr, $n)
{
    $outofplace = -1;
  
    for ($index = 0; $index < $n; $index ++)
    {
        if ($outofplace >= 0)
        {
            // find the item which must be moved 
            // into the out-of-place entry if 
            // out-of-place entry is positive and
            // current entry is negative OR if 
            // out-of-place entry is negative
            // and current entry is negative then 
            // right rotate
            // [...-3, -4, -5, 6...] --> [...6, -3, -4, -5...]
            //     ^                         ^
            //     |                         |
            //     outofplace     -->     outofplace
            //
            if ((($arr[$index] >= 0) && ($arr[$outofplace] < 0)) || 
                (($arr[$index] < 0) && ($arr[$outofplace] >= 0)))
            {
                rightrotate($arr, $n, $outofplace, $index);
  
                // the new out-of-place entry is 
                // now 2 steps ahead
                if ($index - $outofplace > 2)
                    $outofplace = $outofplace + 2;
                else
                    $outofplace = -1;
            }
        }
  
        // if no entry has been flagged out-of-place
        if ($outofplace == -1)
        {
            // check if current entry is out-of-place
            if ((($arr[$index] >= 0) && (!($index & 0x01)))
                || (($arr[$index] < 0) && ($index & 0x01)))
            {
                $outofplace = $index;
            }
        }
    }
}
  
// A utility function to print an
// array 'arr[]' of size 'n'
function printArray(&$arr, $n)
{
    for ($i = 0; $i < $n; $i++)
    echo $arr[$i]." ";
    echo "\n";
}
  
// Driver Code
  
// arr = array(-5, 3, 4, 5, -6, -2, 8, 9, -1, -4);
// arr = array(-5, -3, -4, -5, -6, 2 , 8, 9, 1 , 4);
// arr = array(5, 3, 4, 2, 1, -2 , -8, -9, -1 , -4);
// arr = array(-5, 3, -4, -7, -1, -2 , -8, -9, 1 , -4);
$arr = array(-5, -2, 5, 2, 4, 7, 1, 8, 0, -8);
$n = sizeof($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
?>

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Output:

Given array is
-5 -2 5 2 4 7 1 8 0 -8
Rearranged array is
-5 5 -2 2 -8 4 7 1 8 0

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

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



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