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Move all negative numbers to beginning and positive to end with constant extra space

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  • Difficulty Level : Easy
  • Last Updated : 14 Jun, 2022
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An array contains both positive and negative numbers in random order. Rearrange the array elements so that all negative numbers appear before all positive numbers.

Examples : 

Input: -12, 11, -13, -5, 6, -7, 5, -3, -6
Output: -12 -13 -5 -7 -3 -6 11 6 5

Note: Order of elements is not important here.

Naive approach: The idea is to sort the array of elements, this will make sure that all the negative elements will come before all the positive elements.
Below is the implementation of the above approach:

C++




#include <iostream>
#include<vector>
#include<algorithm>
using namespace std;
void move(vector<int>& arr){
  sort(arr.begin(),arr.end());
}
int main() {
 
    vector<int> arr = { -1, 2, -3, 4, 5, 6, -7, 8, 9 };
      move(arr);
    for (int e : arr)
       cout<<e << " ";
    return 0;
}
 
// This code is contributed by repakaeshwaripriya

Java




// Java program to move all negative numbers to the
// beginning and all positive numbers to the end with
// constant extra space
import java.util.*;
public class Gfg {
    
    public static void move(int[] arr)
    {
        Arrays.sort(arr);
    }
 
    // Driver code
    public static void main(String[] args)
    {
        int[] arr = { -1, 2, -3, 4, 5, 6, -7, 8, 9 };
        move(arr);
        for (int e : arr)
            System.out.print(e + " ");
    }
}
// This article is contributed by aadityapburujwale

Python3




# Python code for the same approach
def move(arr):
  arr.sort()
 
# driver code
arr = [ -1, 2, -3, 4, 5, 6, -7, 8, 9 ]
move(arr)
for e in arr:
    print(e , end = " ")
 
# This code is contributed by shinjanpatra

C#




// C# program to move all negative numbers to the
// beginning and all positive numbers to the end with
// constant extra space
using System;
using System.Collections;
public class Gfg {
    
    public static void move(int[] arr)
    {
        Array.Sort(arr);
    }
 
    // Driver code
    public static void Main()
    {
        int[] arr = { -1, 2, -3, 4, 5, 6, -7, 8, 9 };
        move(arr);
        foreach (int e in arr)
            Console.Write(e + " ");
    }
}
// This code is contributed by Saurabh Jaiswal

Javascript




<script>
 
// JavaScript Code for the same approach
 
function move(arr){
    arr.sort();
}
 
// driver code
   
let arr = [ -1, 2, -3, 4, 5, 6, -7, 8, 9 ];
move(arr);
for (let e of arr)
    document.write(e , " ");
   
// This code is contributed by shinjanpatra
 
</script>

Output

-7 -3 -1 2 4 5 6 8 9 

Time Complexity: O(n*log(n)), Where n is the length of the given array.
Auxiliary Space: O(n)

Efficient Approach 1:
The idea is to simply apply the partition process of quicksort

C++




// A C++ program to put all negative
// numbers before positive numbers
#include <bits/stdc++.h>
using namespace std;
 
void rearrange(int arr[], int n)
{
    int j = 0;
    for (int i = 0; i < n; i++) {
        if (arr[i] < 0) {
            if (i != j)
                swap(arr[i], arr[j]);
            j++;
        }
    }
}
 
// A utility function to print an array
void printArray(int arr[], int n)
{
    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);
}
 
// Driver code
int main()
{
    int arr[] = { -1, 2, -3, 4, 5, 6, -7, 8, 9 };
    int n = sizeof(arr) / sizeof(arr[0]);
    rearrange(arr, n);
    printArray(arr, n);
    return 0;
}

Java




// Java program to put all negative
// numbers before positive numbers
import java.io.*;
 
class GFG {
 
    static void rearrange(int arr[], int n)
    {
        int j = 0, temp;
        for (int i = 0; i < n; i++) {
            if (arr[i] < 0) {
                if (i != j) {
                    temp = arr[i];
                    arr[i] = arr[j];
                    arr[j] = temp;
                }
                j++;
            }
        }
    }
 
    // A 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] + " ");
    }
 
    // Driver code
    public static void main(String args[])
    {
        int arr[] = { -1, 2, -3, 4, 5, 6, -7, 8, 9 };
        int n = arr.length;
 
        rearrange(arr, n);
        printArray(arr, n);
    }
}
 
// This code is contributed by Nikita Tiwari.

Python3




# A Python 3 program to put
# all negative numbers before
# positive numbers
 
def rearrange(arr, n ) :
 
    # Please refer partition() in
    # below post
    # https://www.geeksforgeeks.org / quick-sort / j = 0
    j = 0
    for i in range(0, n) :
        if (arr[i] < 0) :
            temp = arr[i]
            arr[i] = arr[j]
            arr[j]= temp
            j = j + 1
    print(arr)
 
# Driver code
arr = [-1, 2, -3, 4, 5, 6, -7, 8, 9]
n = len(arr)
rearrange(arr, n)
 
 
# This code is contributed by Nikita Tiwari.

C#




// C# program to put all negative
// numbers before positive numbers
using System;
 
class GFG {
    static void rearrange(int[] arr, int n)
    {
 
        int j = 0, temp;
        for (int i = 0; i < n; i++) {
            if (arr[i] < 0) {
                temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
                j++;
            }
        }
    }
 
    // A utility function to print an array
    static void printArray(int[] arr, int n)
    {
        for (int i = 0; i < n; i++)
            Console.Write(arr[i] + " ");
    }
 
    // Driver code
    public static void Main()
    {
        int[] arr = { -1, 2, -3, 4, 5, 6, -7, 8, 9 };
        int n = arr.Length;
 
        rearrange(arr, n);
        printArray(arr, n);
    }
}
 
// This code is contributed by nitin mittal.

PHP




<?php
// A PHP program to put all negative
// numbers before positive numbers
 
function rearrange(&$arr, $n)
{
    $j = 0;
    for ($i = 0; $i < $n; $i++)
    {
        if ($arr[$i] < 0)
        {
            if ($i != $j)
            {
                $temp = $arr[$i];
                $arr[$i] = $arr[$j];
                $arr[$j] = $temp;
            }
            $j++;
        }
    }
}
 
// A utility function to print an array
function printArray(&$arr, $n)
{
    for ($i = 0; $i < $n; $i++)
        echo $arr[$i]." ";
}
 
// Driver code
$arr = array(-1, 2, -3, 4, 5, 6, -7, 8, 9 );
$n = sizeof($arr);
rearrange($arr, $n);
printArray($arr, $n);
 
// This code is contributed by ChitraNayal
?>

Javascript




<script>
// A JavaScript program to put all negative
// numbers before positive numbers
 
function rearrange(arr, n)
{
    let j = 0;
    for (let i = 0; i < n; i++) {
        if (arr[i] < 0) {
            if (i != j){
                let temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
            j++;
        }
    }
}
 
// A utility function to print an array
function printArray(arr, n)
{
    for (let i = 0; i < n; i++)
        document.write(arr[i] + " ");
}
 
// Driver code
    let arr = [ -1, 2, -3, 4, 5, 6, -7, 8, 9 ];
    let n = arr.length;
    rearrange(arr, n);
    printArray(arr, n);
 
 
 
 
// This code is contributed by Surbhi Tyagi.
</script>

Output

-1 -3 -7 4 5 6 2 8 9 

Time complexity: O(N) 
Auxiliary Space: O(1)

Two Pointer Approach: The idea is to solve this problem with constant space and linear time is by using a two-pointer or two-variable approach where we simply take two variables like left and right which hold the 0 and N-1 indexes. Just need to check that :

  1. Check If the left and right pointer elements are negative then simply increment the left pointer.
  2. Otherwise, if the left element is positive and the right element is negative then simply swap the elements, and simultaneously increment and decrement the left and right pointers.
  3. Else if the left element is positive and the right element is also positive then simply decrement the right pointer.
  4. Repeat the above 3 steps until the left pointer ≤ right pointer.

Below is the implementation of the above approach:

C++




// C++ program of the above
// approach
 
#include <iostream>
using namespace std;
 
// Function to shift all the
// negative elements on left side
void shiftall(int arr[], int left,
              int right)
{
   
  // Loop to iterate over the
  // array from left to the right
  while (left<=right)
  {
    // Condition to check if the left
    // and the right elements are
    // negative
    if (arr[left] < 0 && arr[right] < 0)
      left+=1;
     
    // Condition to check if the left
    // pointer element is positive and
    // the right pointer element is negative
    else if (arr[left]>0 && arr[right]<0)
    {
      int temp=arr[left];
      arr[left]=arr[right];
      arr[right]=temp;
      left+=1;
      right-=1;
    }
     
    // Condition to check if both the
    // elements are positive
    else if (arr[left]>0 && arr[right] >0)
      right-=1;
    else{
      left += 1;
      right -= 1;
    }
  }
}
 
// Function to print the array
void display(int arr[], int right){
   
  // Loop to iterate over the element
  // of the given array
  for (int i=0;i<=right;++i){
    cout<<arr[i]<<" ";
  }
  cout<<endl;
}
 
// Driver Code
int main()
{
  int arr[] = {-12, 11, -13, -5,
               6, -7, 5, -3, 11};
  int arr_size = sizeof(arr) /
                sizeof(arr[0]);
   
  // Function Call
  shiftall(arr,0,arr_size-1);
  display(arr,arr_size-1);
  return 0;
}
 
//added by Dhruv Goyal

Java




// Java program of the above
// approach
import java.io.*;
 
class GFG{
 
// Function to shift all the
// negative elements on left side
static void shiftall(int[] arr, int left,
                     int right)
{
     
    // Loop to iterate over the
    // array from left to the right
    while (left <= right)
    {
         
        // Condition to check if the left
        // and the right elements are
        // negative
        if (arr[left] < 0 && arr[right] < 0)
            left++;
 
        // Condition to check if the left
        // pointer element is positive and
        // the right pointer element is negative
        else if (arr[left] > 0 && arr[right] < 0)
        {
            int temp = arr[left];
            arr[left] = arr[right];
            arr[right] = temp;
            left++;
            right--;
        }
 
        // Condition to check if both the
        // elements are positive
        else if (arr[left] > 0 && arr[right] > 0)
            right--;
        else
        {
            left++;
            right--;
        }
    }
}
 
// Function to print the array
static void display(int[] arr, int right)
{
     
    // Loop to iterate over the element
    // of the given array
    for(int i = 0; i <= right; ++i)
        System.out.print(arr[i] + " ");
         
    System.out.println();
}
 
// Drive code
public static void main(String[] args)
{
    int[] arr = { -12, 11, -13, -5,
                   6, -7, 5, -3, 11 };
                    
    int arr_size = arr.length;
 
    // Function Call
    shiftall(arr, 0, arr_size - 1);
    display(arr, arr_size - 1);
}
}
 
// This code is contributed by dhruvgoyal267

Python3




# Python3 program of the
# above approach
 
# Function to shift all the
# the negative elements to
# the left of the array
def shiftall(arr,left,right):
   
  # Loop to iterate while the
  # left pointer is less than
  # the right pointer
  while left<=right:
     
    # Condition to check if the left
    # and right pointer negative
    if arr[left] < 0 and arr[right] < 0:
      left+=1
       
    # Condition to check if the left
    # pointer element is positive and
    # the right pointer element is
    # negative
    else if arr[left]>0 and arr[right]<0:
      arr[left], arr[right] = \
              arr[right],arr[left]
      left+=1
      right-=1
       
    # Condition to check if the left
    # pointer is positive and right
    # pointer as well
    else if arr[left]>0 and arr[right]>0:
      right-=1
    else:
      left+=1
      right-=1
       
# Function to print the array
def display(arr):
  for i in range(len(arr)):
    print(arr[i], end=" ")
  print()
 
# Driver Code
if __name__ == "__main__":
  arr=[-12, 11, -13, -5, \
       6, -7, 5, -3, 11]
  n=len(arr)
  shiftall(arr,0,n-1)
  display(arr)
 
# Sumit Singh

C#




// C# program of the above
// approach
using System.IO;
using System;
class GFG
{
    // Function to shift all the
    // negative elements on left side
    static void shiftall(int[] arr, int left,int right)
    {
       
        // Loop to iterate over the
        // array from left to the right
        while (left <= right)
        {
          
            // Condition to check if the left
            // and the right elements are
            // negative
            if (arr[left] < 0 && arr[right] < 0)
                left++;
  
            // Condition to check if the left
            // pointer element is positive and
            // the right pointer element is negative
            else if (arr[left] > 0 && arr[right] < 0)
            {
                int temp = arr[left];
                arr[left] = arr[right];
                arr[right] = temp;
                left++;
                right--;
            }
  
            // Condition to check if both the
            // elements are positive
            else if (arr[left] > 0 && arr[right] > 0)
                right--;
            else
            {
                left++;
                right--;
            }
        }
    }
   
    // Function to print the array
    static void display(int[] arr, int right)
    {
       
        // Loop to iterate over the element
        // of the given array
        for(int i = 0; i <= right; ++i)
        {
            Console.Write(arr[i] + " ");
             
        }
        Console.WriteLine();
    }
     
    // Drive code                  
    static void Main()
    {
        int[] arr = {-12, 11, -13, -5, 6, -7, 5, -3, 11};
        int arr_size = arr.Length;
        shiftall(arr, 0, arr_size - 1);
        display(arr, arr_size - 1);
    }
}
 
// This code is contributed by avanitrachhadiya2155

Javascript




<script>
 
// Javascript program of the above
// approach
 
// Function to shift all the
// negative elements on left side
function shiftall(arr, left, right)
{
     
    // Loop to iterate over the
    // array from left to the right
    while (left <= right)
    {
         
        // Condition to check if the left
        // and the right elements are
        // negative
        if (arr[left] < 0 && arr[right] < 0)
            left += 1;
         
        // Condition to check if the left
        // pointer element is positive and
        // the right pointer element is negative
        else if(arr[left] > 0 && arr[right] < 0)
        {
            let temp = arr[left];
            arr[left] = arr[right];
            arr[right] = temp;
            left += 1;
            right -= 1;
        }
         
        // Condition to check if both the
        // elements are positive
        else if (arr[left] > 0 && arr[right] > 0)
            right -= 1;
        else
        {
            left += 1;
            right -= 1;
        }
    }
}
 
// Function to print the array
function display(arr, right)
{
     
    // Loop to iterate over the element
    // of the given array
    for(let i = 0; i < right; i++)
        document.write(arr[i] + " ");
}
 
// Driver Code
let arr = [ -12, 11, -13, -5,
             6, -7, 5, -3, 11 ];
let arr_size = arr.length;
 
// Function Call
shiftall(arr, 0, arr_size - 1);
display(arr, arr_size);
  
// This code is contributed by annianni
 
</script>

Output

-12 -3 -13 -5 -7 6 5 11 11 

This is an in-place rearranging algorithm for arranging the positive and negative numbers where the order of elements is not maintained.
Time Complexity: O(N)
Auxiliary Space: O(1)

The problem becomes difficult if we need to maintain the order of elements. Please refer to Rearrange positive and negative numbers with constant extra space for details.

This article is contributed by Apoorva. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeek’s main page and help other Geeks.

Approach 3:
Here, we will use the famous Dutch National Flag Algorithm for two “colors”. The first color will be for all negative integers and the second color will be for all positive integers. We will divide the array into three partitions with the help of two pointers, low and high. 

  1. ar[1…low-1] negative integers
  2. ar[low…high] unknown
  3. ar[high+1…N] positive integers

Now, we explore the array with the help of low pointer, shrinking the unknown partition, and moving elements to their correct partition in the process. We do this until we have explored all the elements, and size of the unknown partition shrinks to zero.

Below is the implementation of the above approach:

C++




#include <iostream>
using namespace std;
 
// Swap Function.
void swap(int &a,int &b){
  int temp =a;
  a=b;
  b=temp;
}
   
// Using Dutch National Flag Algorithm.
void reArrange(int arr[],int n){
      int low =0,high = n-1;
      while(low<high){
      if(arr[low]<0){
          low++;
      }else if(arr[high]>0){
          high--;
      }else{
        swap(arr[low],arr[high]);
      }
    }
}
void displayArray(int arr[],int n){
  for(int i=0;i<n;i++){
    cout<<arr[i]<<" ";
  }
  cout<<endl;
}
int main() {
    // Data
    int arr[] = {1, 2,  -4, -5, 2, -7, 3, 2, -6, -8, -9, 3, 2,  1};
      int n = sizeof(arr)/sizeof(arr[0]);
      reArrange(arr,n);
    displayArray(arr,n);
    return 0;
}

Java




// Java program to move all negative numbers to the
// beginning and all positive numbers to the end with
// constant extra space
 
public class Gfg {
 
    // a utility function to swap two elements of an array
    public static void swap(int[] ar, int i, int j)
    {
        int t = ar[i];
        ar[i] = ar[j];
        ar[j] = t;
    }
 
    // function to shilf all negative integers to the left
    // and all positive integers to the right
    // using Dutch National Flag Algorithm
    public static void move(int[] ar)
    {
        int low = 0;
        int high = ar.length - 1;
        while (low <= high) {
            if (ar[low] <= 0)
                low++;
            else
                swap(ar, low, high--);
        }
    }
 
    // Driver code
    public static void main(String[] args)
    {
        int[] ar = { 1, 2,  -4, -5, 2, -7, 3,
                     2, -6, -8, -9, 3, 21 };
        move(ar);
        for (int e : ar)
            System.out.print(e + " ");
    }
}
 
// This code is contributed by Vedant Harshit

Python3




# Python code for the approach
 
# Using Dutch National Flag Algorithm.
def reArrange(arr, n):
    low,high = 0, n - 1
    while(low<high):
        if(arr[low] < 0):
            low += 1
        elif(arr[high] > 0):
            high -= 1
        else:
            arr[low],arr[high] = arr[high],arr[low]
 
def displayArray(arr, n):
 
    for i in range(n):
        print(arr[i],end = " ")
   
    print('')
 
# driver code
# Data
arr = [1, 2-4, -5, 2, -7, 3, 2, -6, -8, -9, 3, 21]
n = len(arr)
reArrange(arr,n)
displayArray(arr,n)
 
# This code is contributed by shinjanpatra

C#




// Include namespace system
using System;
 
// C# program to move all negative numbers to the
// beginning and all positive numbers to the end with
// constant extra space
public class Gfg
{
  // a utility function to swap two elements of an array
  public static void swap(int[] ar, int i, int j)
  {
    var t = ar[i];
    ar[i] = ar[j];
    ar[j] = t;
  }
  // function to shilf all negative integers to the left
  // and all positive integers to the right
  // using Dutch National Flag Algorithm
  public static void move(int[] ar)
  {
    var low = 0;
    var high = ar.Length - 1;
    while (low <= high)
    {
      if (ar[low] <= 0)
      {
        low++;
      }
      else
      {
        Gfg.swap(ar, low, high--);
      }
    }
  }
 
  // Driver code
  public static void Main(String[] args)
  {
    int[] ar = {1, 2, -4, -5, 2, -7, 3, 2, -6, -8, -9, 3, 2, 1};
    Gfg.move(ar);
    foreach (int e in ar)
    {            Console.Write(e.ToString() + " ");
    }
  }
}
 
// This code is contributed by mukulsomukesh

Javascript




<script>
 
// JavaScript code for the approach
 
// Using Dutch National Flag Algorithm.
function reArrange(arr, n){
    let low = 0,high = n - 1
    while(low<high){
        if(arr[low] < 0)
            low += 1
        else if(arr[high] > 0)
            high -= 1
        else{
            let temp = arr[low]
            arr[low] = arr[high]
            arr[high] = temp
        }
    }
}
 
function displayArray(arr, n){
 
    for(let i = 0; i < n; i++){
        document.write(arr[i]," ")
    }
    document.write('</br>')
}
 
// driver code
// Data
let arr = [1, 2,  -4, -5, 2, -7, 3, 2, -6, -8, -9, 3, 2,  1]
let n = arr.length
reArrange(arr,n)
displayArray(arr,n)
 
// This code is contributed by shinjanpatra
 
</script>

Output

-9 -8 -4 -5 -6 -7 3 2 2 2 1 3 2 1 

Time complexity: O(N) 
Auxiliary Space: O(1)

The order of elements does not matter here. Explanation and code contributed by Vedant Harshit


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