Smallest Positive missing number

Medium

You are given an array arr[] of N integers including 0. The task is to find the smallest positive number missing from the array.

Example 1:

Input:
N = 5
arr[] = {1,2,3,4,5}
Output: 6
Explanation: Smallest positive missing 
number is 6.

Example 2:

Input:
N = 5
arr[] = {0,-10,1,3,-20}
Output: 2
Explanation: Smallest positive missing 
number is 2.

Your Task:
The task is to complete the function missingNumber() which returns the smallest positive missing number in the array.

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

Constraints:
1 <= N <= 10⁶
-10⁶ <= arr[i] <= 10⁶