Create pairs.java

Author: andy489

additional/arrays/pairs.java

@@ -0,0 +1,96 @@
+import java.util.Arrays;
+import java.util.Scanner;
+
+public class Problem11Pairs { // O(N * log(N)) (linear) solution for counting (without the printing)
+    public static void main(String... args) {
+        Scanner sc = new Scanner(System.in);
+
+        System.out.print("Target sum: ");
+        int target = Integer.parseInt(sc.nextLine());
+
+        System.out.print("Integers: ");
+        String[] input = sc.nextLine().split(" ");
+        final int N = input.length;
+
+        int[] arr = new int[N];
+
+        for (int i = 0; i < N; i++) {
+            arr[i] = Integer.parseInt(input[i]);
+        }
+
+        Arrays.sort(arr);
+
+        long pairs = 0;
+        int[] uniques = new int[N];
+        int[] repetitions = new int[N];
+
+        int k = 0;
+        int rep = 1;
+        for (int i = 1; i < N; i++) {
+            if (arr[i] == arr[i - 1]) {
+                rep++;
+                if (i == N - 1) {
+                    uniques[k] = arr[i];
+                    repetitions[k] = rep;
+                    k++;
+                }
+            } else {
+                uniques[k] = arr[i - 1];
+                repetitions[k] = rep;
+                k++;
+                rep = 1;
+                if (i == N - 1) {
+                    uniques[k] = arr[i];
+                    repetitions[k] = 1;
+                    k++;
+                }
+            }
+        }
+
+        int l = 0;
+        int r = k - 1;
+        if (l == r) {
+            if (uniques[l] * 2 == target) {
+                int combinations = repetitions[l] * (repetitions[l] - 1) / 2;
+                pairs += combinations;
+//                for (int i = 0; i < combinations; i++) {
+//                    System.out.printf("%d, %d%n", uniques[l], uniques[l]);
+//                }
+            }
+        } else {
+            while (l < r) {
+                if (uniques[l] * 2 == target) {
+                    int combinations = repetitions[l] * (repetitions[l] - 1) / 2;
+                    pairs += combinations;
+//                    for (int i = 0; i < combinations; i++) {
+//                        System.out.printf("%d, %d%n", uniques[l], uniques[l]);
+//                    }
+                }
+
+                if (uniques[r] * 2 == target) {
+                    int combinations = repetitions[r] * (repetitions[r] - 1) / 2;
+                    pairs += combinations;
+//                    for (int i = 0; i < combinations; i++) {
+//                        System.out.printf("%d, %d%n", uniques[r], uniques[r]);
+//                    }
+                }
+
+                int currentCombination = uniques[l] + uniques[r];
+                if (currentCombination == target) {
+                    int combinations = repetitions[l] * repetitions[r];
+                    pairs += combinations;
+//                    for (int i = 0; i < combinations; i++) {
+//                        System.out.printf("%d, %d%n", uniques[l], uniques[r]);
+//                    }
+                    l++;
+                } else if (currentCombination < target) {
+                    l++;
+                } else {
+                    r--;
+                }
+            }
+        }
+
+        System.out.printf("Total pairs: %d", pairs);
+    }
+}