Arrange an Array in Alternate Positive Negative Elements Using Java

7 May 2025 | 5 min read

Sorting things in an array looks for arrangement of objects in an array is one of the basic problem type prevalent in computer science for applications in pattern matching techniques and simulations or data graphics and visualizations.

One such task is to sort elements of some numerical array in the way that shifting from the positive number to the negative one would be impossible. This kind of rearrangement has to provide the greatest amount of flexibility while keeping the relative order of elements as little shuffled as possible.

Problem Statement

Suppose you are given an array of integers, you should reorganize the integers in the array in a way such that the positive and the negative numbers are adjacent. When the positive or the negative values are in number than the other then the remaining values are at the last of the array.

Example:

Approach 1: Rearrange in Place - Brute Force

Traverse the array and, for each element check whether it violates the rule of the alternation.

If an element is misplaced (it is positive where negative is required or a negative value is required where a positive value is required) look for a correct element and then swap elements in order to bring it up.

Steps:

Traverse the array.
If one has been misplaced, look around for the next one that will have an opposite sign to it.
Swap operations to move necessary digits to obtain a required signed number in its rank.

Approach 2: Two Pass Solution (Extra Space)

Add more space to store positive and negative numbers in a different storages.

The first approach to combine the two lists alternatively into the result array is called the merge operation.

Steps:

Iterate through the array and store positives and negatives each in their respective lists.
Join the lists in turn into the result list.If one of the lists is odd, then the last element of the list is added to the result array.

Approach 3: Optimal Approach (In-Place Rearrangement Using Two Pointers)

Sort the given array in-situ using two-pointer approach and without using any additional space.

While acceptor pointer locates lost elements, other pointer looks for the next element with a correct sign for the swap.

Steps:

Reserve two movement indices - one for searching misplaced numbers and another for searching numbers to interchange.
If thought to be incorrectly placed, the subarray is rotated so that the next element with the correct sign is placed at the correct location.

File Name: OptimalAlternateRearrange.java

Output:

 
Original Array:
1 2 -3 -4 -5 6 
Rearranged Array:
-3 1 -4 2 -5 6

Complexity Analysis

Time Complexity: O(n) for the optimal approach, as each element is traversed and rotated efficiently.

Space Complexity: O(1) for the optimal solution since no extra space is used.

Conclusion

Sorting an array in such a way that every element starting from the first element alternating in sign with the next step can be attained by brute force, using space, or in optimum using the space.

The best result utilizes the two-pointer technique along with subarray rotation leading to a linear time complexity with no extra space complexity. It makes a check on the computer as it shall perform efficiently no matters the size of the arrays that are the inputs.

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