Finds the two missing numbers in the given array which contains numbers from 1 to n inclusive except two numbers

MissingTwoNumbers.java

/* File:    MissingTwoNumbers.java
 * Created: 6/6/2018
 * Author:  Sabbir Manandhar
 *
 * Copyright (c) 2018 WorldLingo Inc.
 */

/**
 * @author Sabbir Manandhar (lastname)(dot)(firstname)(at)gmail(dot)com
 * @version 1.0
 */
public class MissingTwoNumbers {

  public static void main(String[] args) {
    int[] input = new int[]{10, 9, 5, 6, 8, 3, 7, 4, 11, 1, 2, 13, 15};
    int[] res = find(input);
    System.out.println(res[0] + " " + res[1]);
  } // main

  //-------------------------------------------------------------------------------

  /**
   * Finds the two missing numbers in the array
   * @param input input array containing numbers [1,n] except two numbers
   * @return array of two missing numbers
   */
  public static int[] find(int[] input) {
    long sum = findSumOfMissingNumbers(input); // find sum of missing numbers
    int pivot = (int) sum / 2;

    // Compute the missing number in lower half
    int lowerHalfXOR = 0;
    for (int i = 1; i <= pivot; i++) {
      lowerHalfXOR ^= i;
    }

    for (int i = 0; i < input.length; i++) {
      if (input[i] <= pivot) {
        lowerHalfXOR ^= input[i];
      }
    }

    // Compute the missing number in upper half
    int upperHalfXOR = 0;
    for (int i = pivot + 1; i <= input.length + 2; i++) {
      upperHalfXOR ^= i;
    }

    for (int i = 0; i < input.length; i++) {
      if (input[i] > pivot) {
        upperHalfXOR ^= input[i];
      }
    }

    return new int[]{lowerHalfXOR, upperHalfXOR};
  } // find

  //-------------------------------------------------------------------------------

  /**
   * Finds the sum of the two missing numbers in the input array
   * @param input array containing numbers [1,n] except two numbers
   * @return sum of the two missing numbers
   */
  public static long findSumOfMissingNumbers(int[] input) {
    int n = input.length + 2;
    int expectedSum = (n * (n+1)) / 2;

    int actualSum = 0;
    for (int num : input) {
      actualSum += num;
    }

    return expectedSum - actualSum;
  } // findSumOfMissingNumbers

  //-------------------------------------------------------------------------------

} // MissingTwoNumbers