Given an array of size N, it contains all the numbers from 1 to N+1 inclusive, except one number. You have to find the missing number.

find_missing.cpp

#include <iostream>
#include <vector>
using namespace std;

int findMissingNumber(int n) {
    int sum = 0, x;
    for(int i = 0; i < n; ++i, sum += x) 
        cin >> x;
    int expected_sum = ((n+1)*(n+2)) / 2;
    return expected_sum - sum;
}

int main() {
    int t;  cin >> t;
    while(t--) {
        int n;  cin >> n;
        cout << findMissingNumber(n) << "\n";
    }
    return 0;
}