# 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++

 `// C++ program to rearrange array in alternating ` `// positive & negative items with O(1) extra space ` `#include ` `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); ` ` `  `    ``cout << ``"Given array is \n"``; ` `    ``printArray(arr, n); ` ` `  `    ``rearrange(arr, n); ` ` `  `    ``cout << ``"Rearranged array is \n"``; ` `    ``printArray(arr, n); ` ` `  `    ``return` `0; ` `} `

## Java

 `//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 `

## Python3

 `# 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 `

## C#

 `// 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 `

## PHP

 ` 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 ` `?> `

Output:

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

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