Find_Missing_And_Repeating.cpp

/*
Problem Statement:
------------------
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)
*/

// Link --> https://practice.geeksforgeeks.org/problems/find-missing-and-repeating2512/1#

// Code:
class Solution
{
public:
    int *findTwoElement(int *a , int n) 
    {
        int *result = new int[2];
        
        // calculating the repetitive element
        for(int i=0 ; i<n ; i++) 
        {
            if(a[abs(a[i]) - 1] > 0)
                a[abs(a[i]) - 1] = -a[abs(a[i]) - 1];
            else
                result[0] = abs(a[i]);
        }
        
        // calculating least positive element
        for(int i=0 ; i<n ; i++) 
            if(a[i] > 0)
                result[1] = i + 1;
        
        return result;
    }
};