穷举法

#include <iostream>
#include <unordered_set>

using namespace std;
int n;
unordered_set<int> s;

int main(void) {
  cin >> n;
  while (n--) {
    int x;
    cin >> x;
    s.emplace(x);
  }

  for (int i = 1; i <= 1e5; ++i)
    if (!s.count(i)) return printf("%d", i), 0;

  return printf("%d", 100001), 0;
}