Find Missing And Repeating.cpp

/*
Find Missing And Repeating
==========================

Given an unsorted array Arr of size N of positive integers. One number 'A' from set {1, 2, …N} is missing and one number 'B' occurs twice in array. Find these two numbers.

Example 1:
Input:
N = 2
Arr[] = {2, 2}
Output: 2 1
Explanation: Repeating number is 2 and 
smallest positive missing number is 1.

Example 2:
Input:
N = 3
Arr[] = {1, 3, 3}
Output: 3 2
Explanation: Repeating number is 3 and 
smallest positive missing number is 2.
Your Task:
You don't need to read input or print anything. Your task is to complete the function findTwoElement() which takes the array of integers arr and n as parameters and returns an array of integers of size 2 denoting the answer ( The first index contains B and second index contains A.)

Expected Time Complexity: O(N)
Expected Auxiliary Space: O(1)

Constraints:
1 ≤ N ≤ 105
1 ≤ Arr[i] ≤ N
*/

int *findTwoElement(int *arr, int n)
{
  int *ans = new int[2];
  for (int i = 0; i < n; i++)
    if (arr[abs(arr[i]) - 1] > 0)
      arr[abs(arr[i]) - 1] *= -1;
    else
      ans[0] = abs(arr[i]);

  for (int i = 0; i < n; i++)
    if (arr[i] > 0)
      ans[1] = i + 1;
  return ans;
}