import java.util.HashMap;
import java.util.Map;
 
public class Main {
    public static int countPairsOptimized(int[] b, int k) {
        int count = 0;
        Map<Integer, Integer> freqMap = new HashMap<>();
        for (int i = b.length - 1; i >= 0; --i) {
            int target = b[i] - k; // 🔥 check future j’s
            if (freqMap.containsKey(target)) {
                count += freqMap.get(target);
            }
            freqMap.put(b[i], freqMap.getOrDefault(b[i], 0) + 1);
        }
        return count;
    }
 
    public static void main(String[] args) {
        int[] b = {1, 5, 3, 4, 2};
        int k = 2;
        System.out.println("Count of pairs: " + countPairsOptimized(b, k));
    }
}

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