Find the MEX of given Array

You are given an array arr[] of size N, the task is to determine the MEX of the array.

MEX is the smallest whole number that is not present in the array.

Examples:

Input: arr[] = {1, 0, 2, 4}
Output: 3
Explanation: 3 is the smallest whole number that is not present in the array

Input: arr[] = {-1, -5, 0, 4}
Output: 1
Explanation: 1 is the smallest whole number that is missing in the array.

Approach: Follow the below idea to solve the problem

Sort the array in increasing order and find the first number starting from 0 which is not present in the array.

Follow the steps to solve the problem:

• Sort the array.
• Initialize a variable mex = 0.
• Travel over the array from index 0 to N-1.
• If the element is equal to mex
  • Increment mex by 1 i.e., mex = mex + 1
  • Else continue the iteration
• Return mex as the final answer.

Below is the implementation of the above approach:

3

Time Complexity: O(N * logN)
Auxiliary Space: O(1)