Place array in alternating positive & negative items with O(1) extra space - Part 2

Software Development
#1

Efficient Approach :
We have already discussed a O(n2) solution that maintains the order of appearance in the array. 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.

// 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 = 0 , j = n - 1 ;

// shift all negative values to the end

while (i < j)

{

while (i <= n - 1 && arr[i] > 0 )

i += 1 ;

while (j >= 0 && arr[j] < 0 )

j -= 1 ;

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);

}

}

Output:

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

Time Complexity : O(N)

Space Complexity : O(1)