Alternate Positives and Negatives

medium Arrays

The array has the same number of positive and negative numbers (and no zero). Rearrange it so the signs alternate, starting with a positive number. The positive numbers must keep their original order, and so must the negative numbers. Print the new array.

Constraints

2 <= n <= 10^5
n is even
no zeros

Examples

Input

6
3 1 -2 -5 2 -4

Output

3 -2 1 -5 2 -4

Stable rearrangement
Two write pointers

Solutions

Best approachTime O(n)Space O(n)

The final place of every number is known: positives take the even places in order, negatives take the odd places in order. A single pass with two write positions is enough.

Same code works in Java810172125Basic features only, so nothing changes between versions.

/*
 * Problem : Alternate the signs.
 * Input   : n and the numbers.
 * Output  : The new array.
 * Approach: Place by index
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;
import java.util.Arrays;
import java.util.stream.Collectors;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static int[] solve(int[] a) {
        // Step 1: Positive numbers go to the even places 0, 2, 4, ... and negative numbers to the odd places 1, 3, 5, ...
        int[] result = new int[a.length];
        int positiveAt = 0;
        int negativeAt = 1;
        // Step 2: One pass from left to right keeps the order of each group.
        for (int value : a) {
            if (value > 0) {
                result[positiveAt] = value;
                positiveAt += 2;
            } else {
                result[negativeAt] = value;
                negativeAt += 2;
            }
        }
        return result;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int size = scanner.nextInt();
        int[] a = new int[size];
        for (int i = 0; i < size; i++) {
            a[i] = scanner.nextInt();
        }
        System.out.println(Arrays.stream(solve(a)).mapToObj(String::valueOf).collect(Collectors.joining(" ")));
    }
}

Hints (1)

Where does the 3rd positive number go?