usaco-practice/cses/Missing Number/index.cpp

#include <iostream>
using namespace std;
long findMissingNo(long arr[], long len) {
  long temp;
  temp = ((len) * (len + 1)) / 2;
  for (long i = 0; i < len - 1; i++)
    temp -= arr[i];
  return temp;
}

int main() {
  long n;
  cin >> n;
  long arr[n];
  for (long i = 0; i < n - 1; i++) {
    cin >> arr[i];
  }
  long missingNo = findMissingNo(arr, n);
  cout << missingNo;
  return 0;
}