dfuoop - Online Java Compiler & Debugging Tool

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static int countValidPairs(int[] arr, int target) {
        int n = arr.length;

        // Split into positive, negative, and zero buckets
        List<Integer> pos = new ArrayList<>();
        List<Integer> neg = new ArrayList<>();
        int zeroCount = 0;

        for (int x : arr) {
            if (x > 0) pos.add(x);
            else if (x < 0) neg.add(x);
            else zeroCount++;
        }

        Collections.sort(pos);
        Collections.sort(neg); // will be increasing (more negative to less negative)

        int count = 0;

        // ---------- Case 1: Both non-negative ----------
        if (target % 2 == 0) {
            int half = target / 2;
            int freqHalf = 0;

            for (int x : pos) {
                if (x == half) freqHalf++;
            }
            if (zeroCount > 0 && half == 0) freqHalf += zeroCount; // include zeros

            // All pairs (half, half)
            count += (freqHalf * (freqHalf - 1)) / 2;
        }

        // ---------- Case 2: Both negative ----------
        if (target % 2 == 0) {
            int half = -target / 2;
            int freqHalf = 0;

            for (int x : neg) {
                if (x == half) freqHalf++;
            }

            // All pairs (half, half)
            count += (freqHalf * (freqHalf - 1)) / 2;
        }

        // ---------- Case 3: One positive, one negative ----------
        if (target % 2 == 0) {
            int half = target / 2;

            // Positive side: a[i] = half
            int freqHalfPos = 0;
            for (int x : pos) if (x == half) freqHalfPos++;

            // Negative side: |a[j]| <= half
            int validNeg = 0;
            for (int x : neg) if (Math.abs(x) <= half) validNeg++;

            count += freqHalfPos * validNeg;

            // Negative side: a[j] = -half
            int freqHalfNeg = 0;
            for (int x : neg) if (x == -half) freqHalfNeg++;

            // Positive side: |a[i]| < half
            int validPos = 0;
            for (int x : pos) if (Math.abs(x) < half) validPos++;

            count += freqHalfNeg * validPos;
        }

        // ---------- Case 4: One negative, one non-negative ----------
        // Already covered in Case 3 (since unordered pairs handled once)

        return count;
    }

    // Driver code
    public static void main(String[] args) {
        int[] arr = {2, -2, 4, -4, 3, -3, 0, 0};
        int target = 6;

        int result = countValidPairs(arr, target);
        System.out.println("Number of valid unordered pairs: " + result);
    }
}