一道需要细细品味的题

#include<iostream>
using namespace std;
#define ll long long
const int N = 1e5+9;
int a[1005];
bool ed[1000005];
int main() 
{
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++)
    {
    cin >> a[i];
  }
  for (int i = 1; i <= n; i++) 
    {
    int now = 0;
    for (int k = i; k <= n; k++) 
        {
      now = now * 10 + a[k];
      if (now > 1e6)break;
      ed[now] = 1;
    }
  }
  for (int i = 0; i <= 1e6; i++) 
    {
    if (!ed[i]) 
        {
      cout << i;
      break;
    }
  }
  return 0;
}