class Solution {
public:
    int missingNumber(vector<int>& arr) {
        int n = arr.size();
        int exsum = n*(n+1)/2;
        int sum = 0;
        for(int i =0 ; i< n ; i++){
            sum+= arr[i];
        }

        return exsum - sum;
    }
};

https://leetcode.com/problems/missing-number/

https://takeuforward.org/arrays/find-the-missing-number-in-an-array/